1 00:00:16,492 --> 00:00:17,950 ADAM MARTIN: And today, we're going 2 00:00:17,950 --> 00:00:23,250 to talk about immunity, which is important, especially 3 00:00:23,250 --> 00:00:26,280 at this time of the year. 4 00:00:26,280 --> 00:00:39,210 So immunity is the resistance to disease 5 00:00:39,210 --> 00:00:41,310 based on a prior exposure. 6 00:00:44,640 --> 00:00:48,400 Based on prior exposure. 7 00:00:51,870 --> 00:00:57,100 And of course, this is the principle behind vaccination. 8 00:00:57,100 --> 00:01:01,170 So humans have been sort of using 9 00:01:01,170 --> 00:01:03,690 the properties of the immune system 10 00:01:03,690 --> 00:01:08,970 to prevent themselves from getting disease for centuries. 11 00:01:08,970 --> 00:01:13,170 One of the first very clear examples of this 12 00:01:13,170 --> 00:01:17,550 is back in the 18th century with the English physician, Edward 13 00:01:17,550 --> 00:01:25,440 Jenner, and Edward Jenner found out or came to the realization 14 00:01:25,440 --> 00:01:30,210 that farm hands on farms, specifically 15 00:01:30,210 --> 00:01:36,690 milk maids, that were exposed to a variant of smallpox 16 00:01:36,690 --> 00:01:39,630 from cows, which is known as cow pox, 17 00:01:39,630 --> 00:01:42,510 could become immune to smallpox. 18 00:01:42,510 --> 00:01:48,450 So cow pox is a less severe form of the disease. 19 00:01:48,450 --> 00:01:53,100 And what Jenner did was to take pustules 20 00:01:53,100 --> 00:01:58,710 from individuals who had the cow pox disease 21 00:01:58,710 --> 00:02:02,340 and inject them into an eight-year-old boy, 22 00:02:02,340 --> 00:02:04,950 and then infect that boy with smallpox 23 00:02:04,950 --> 00:02:11,430 to show that the boy was immune to smallpox after having 24 00:02:11,430 --> 00:02:15,210 received the cow pox material. 25 00:02:15,210 --> 00:02:17,670 And so this is the first example of the vaccine. 26 00:02:20,570 --> 00:02:25,740 And because the vaccine was derived from basically someone 27 00:02:25,740 --> 00:02:28,500 with cow pox, the word vaccine is 28 00:02:28,500 --> 00:02:32,610 from the Latin root of vaca, which is cow, 29 00:02:32,610 --> 00:02:36,840 so that's where the word vaccine comes from, OK? 30 00:02:36,840 --> 00:02:39,840 So today, we're going to talk about the systems 31 00:02:39,840 --> 00:02:45,660 that our bodies have to fight disease 32 00:02:45,660 --> 00:02:49,200 and there are several different all levels 33 00:02:49,200 --> 00:02:51,970 of the immune system. 34 00:02:51,970 --> 00:02:54,460 So I'm going to talk about two of them. 35 00:02:54,460 --> 00:02:59,390 So I'll talk about two levels of immunity. 36 00:03:05,290 --> 00:03:08,080 The first I want to mention is the one 37 00:03:08,080 --> 00:03:10,940 that we're all just like born with, 38 00:03:10,940 --> 00:03:12,670 which is known as innate immunity. 39 00:03:16,037 --> 00:03:16,999 [SNEEZING] 40 00:03:18,923 --> 00:03:20,380 [SNEEZING] 41 00:03:20,380 --> 00:03:23,140 Bless you. 42 00:03:23,140 --> 00:03:27,170 So innate immunity, as the name implies, 43 00:03:27,170 --> 00:03:30,295 is something that we are born with, so this is inborn. 44 00:03:37,440 --> 00:03:42,300 It also doesn't have a delay in when it's activated, right? 45 00:03:42,300 --> 00:03:45,330 So if you have an infection, this 46 00:03:45,330 --> 00:03:47,750 is sort of the first line of defense, right? 47 00:03:47,750 --> 00:03:53,310 There is an immediate response, and that's the innate immune 48 00:03:53,310 --> 00:03:55,155 system, so this is immediate. 49 00:03:55,155 --> 00:03:57,060 Here, I'll put this down here. 50 00:03:57,060 --> 00:03:57,900 It's immediate. 51 00:04:03,260 --> 00:04:09,180 So one example is of an innate immune response. 52 00:04:09,180 --> 00:04:11,810 This is not in the body but ex vivo, 53 00:04:11,810 --> 00:04:14,330 but here you see a human neutrophil, 54 00:04:14,330 --> 00:04:17,690 and neutrophils are part of our innate immune system. 55 00:04:17,690 --> 00:04:21,260 And neutrophils hunt and kill bacteria, right? 56 00:04:21,260 --> 00:04:25,820 You see that neutrophil chasing after that bacterium, 57 00:04:25,820 --> 00:04:27,080 and it's going for it. 58 00:04:27,080 --> 00:04:29,780 It's really trying to get it, but that bacteria really 59 00:04:29,780 --> 00:04:31,750 wants to get away, but got it! 60 00:04:31,750 --> 00:04:32,720 OK, great. 61 00:04:32,720 --> 00:04:37,380 So these neutrophils are part of the innate immune system. 62 00:04:37,380 --> 00:04:39,680 It's inborn, it's immediate. 63 00:04:39,680 --> 00:04:44,060 And in addition, the response of the innate immune system 64 00:04:44,060 --> 00:04:47,780 doesn't really change if you've been 65 00:04:47,780 --> 00:04:55,760 exposed to an infectious agent prior to the-- 66 00:04:55,760 --> 00:05:02,450 OK, so this does not change. 67 00:05:02,450 --> 00:05:05,120 I'm using the Greek delta for change. 68 00:05:05,120 --> 00:05:11,580 Does not change with prior exposure. 69 00:05:11,580 --> 00:05:14,900 OK, so it's sort of like a constant surveillance 70 00:05:14,900 --> 00:05:20,180 mechanism in your body that will go after foreign agents, OK? 71 00:05:20,180 --> 00:05:23,150 Now, this is very different from the next level 72 00:05:23,150 --> 00:05:28,580 of immunity, which is known as adaptive immunity. 73 00:05:36,350 --> 00:05:40,610 And as the name adaptive immunity implies, 74 00:05:40,610 --> 00:05:43,640 this is a type of immunity that does change. 75 00:05:43,640 --> 00:05:45,920 It adapts, OK? 76 00:05:45,920 --> 00:05:50,250 And this type of immunity is acquired, 77 00:05:50,250 --> 00:05:52,700 so it's also known as acquired immunity, 78 00:05:52,700 --> 00:06:01,970 but it's acquired with exposure to a foreign agent, OK? 79 00:06:01,970 --> 00:06:07,430 So this involves a change in immunity, this one does not, 80 00:06:07,430 --> 00:06:10,310 but the innate immune response is immediate, 81 00:06:10,310 --> 00:06:13,490 whereas adaptive immunity takes time. 82 00:06:13,490 --> 00:06:15,485 There's a delay, so this is also delayed. 83 00:06:23,870 --> 00:06:27,560 It's also highly specific, OK? 84 00:06:27,560 --> 00:06:30,920 So it's highly specific to the foreign agents 85 00:06:30,920 --> 00:06:34,610 that you are infected with. 86 00:06:34,610 --> 00:06:37,300 The innate immune system is less specific. 87 00:06:37,300 --> 00:06:40,190 It'll recognize, like, things like bacteria, 88 00:06:40,190 --> 00:06:43,280 but it won't be able to necessarily distinguish 89 00:06:43,280 --> 00:06:46,890 between different types of bacteria. 90 00:06:46,890 --> 00:06:50,520 So this is more specific than the innate immune system. 91 00:06:50,520 --> 00:06:53,750 This is why every year, you have to get a flu shot, 92 00:06:53,750 --> 00:06:56,660 because the flu virus is constantly changing. 93 00:06:56,660 --> 00:06:59,150 And our immune system is so specific 94 00:06:59,150 --> 00:07:02,060 that unless we get a new vaccination, 95 00:07:02,060 --> 00:07:07,280 our bodies will not be able to recognize it, OK? 96 00:07:07,280 --> 00:07:12,230 So this is-- so now I'm going to break down adaptive immunity 97 00:07:12,230 --> 00:07:13,175 into two branches. 98 00:07:17,310 --> 00:07:26,510 One is known as humoral immunity, 99 00:07:26,510 --> 00:07:40,370 and humoral immunity is basically protein-mediated, 100 00:07:40,370 --> 00:07:42,020 and there are proteins that mediate 101 00:07:42,020 --> 00:07:44,480 this are called antibodies. 102 00:07:49,350 --> 00:07:59,350 These antibodies are proteins, and it's called humoral 103 00:07:59,350 --> 00:08:02,080 because the antibodies can be secreted 104 00:08:02,080 --> 00:08:05,860 into the fluids or humors of our body, which is basically 105 00:08:05,860 --> 00:08:07,870 the blood, OK? 106 00:08:07,870 --> 00:08:09,790 So there is humoral immunity. 107 00:08:09,790 --> 00:08:12,160 The other type of adaptive immunity 108 00:08:12,160 --> 00:08:20,100 is cell mediated, and one thing I 109 00:08:20,100 --> 00:08:24,960 want to point out that the types of cells that make an antibody 110 00:08:24,960 --> 00:08:25,965 are known as B cells. 111 00:08:29,520 --> 00:08:32,250 What the B stands for isn't really important, 112 00:08:32,250 --> 00:08:34,980 but one thing that's helpful is that these cells 113 00:08:34,980 --> 00:08:44,670 mature in the bone marrow, and B stands for bone marrow, OK? 114 00:08:44,670 --> 00:08:46,590 So you can always remember where they mature. 115 00:08:49,800 --> 00:08:53,650 Now cell-mediated immunity, in contrast, 116 00:08:53,650 --> 00:08:57,780 involves a different type of cell called a T cell, 117 00:08:57,780 --> 00:09:00,690 and the T of T cell stands for thymus 118 00:09:00,690 --> 00:09:02,625 because these cells mature in the thymus. 119 00:09:11,760 --> 00:09:17,410 And I just want to point out where these cells come from. 120 00:09:17,410 --> 00:09:21,970 So we talked about adult stem cells earlier, 121 00:09:21,970 --> 00:09:26,590 and in this case, these T and B lymphocytes over here 122 00:09:26,590 --> 00:09:30,340 are derived from a multipotent hematopoietic stem 123 00:09:30,340 --> 00:09:33,970 cell, which generates a whole bunch 124 00:09:33,970 --> 00:09:35,900 of different types of cells. 125 00:09:35,900 --> 00:09:39,580 Many of them are involved in the innate immune response, 126 00:09:39,580 --> 00:09:43,450 but this common lymphoid progenitor over here 127 00:09:43,450 --> 00:09:45,040 gives rise to D-- 128 00:09:45,040 --> 00:09:47,890 T lymphocytes and B lymphocytes, which are 129 00:09:47,890 --> 00:09:52,060 involved in adaptive immunity. 130 00:09:52,060 --> 00:09:54,960 OK, so it's not important that you remember where-- 131 00:09:54,960 --> 00:09:58,690 what all these cells come from or what they, like, 132 00:09:58,690 --> 00:10:03,220 what the tree is, but that these cells arise 133 00:10:03,220 --> 00:10:07,030 from a common progenitor cell. 134 00:10:07,030 --> 00:10:14,080 OK, so both of these branches of the adaptive immune system 135 00:10:14,080 --> 00:10:19,260 have what are known as antigen receptors. 136 00:10:19,260 --> 00:10:23,470 I'll abbreviate antigen, AG. 137 00:10:23,470 --> 00:10:27,220 So they have antigen receptors, meaning 138 00:10:27,220 --> 00:10:29,680 that they have things on them that 139 00:10:29,680 --> 00:10:34,510 recognize specific antigens, and antigens are basically 140 00:10:34,510 --> 00:10:37,510 things that result in an immune response. 141 00:10:37,510 --> 00:10:39,190 They could be proteins. 142 00:10:39,190 --> 00:10:54,150 Antigens are substances that activate the immune system. 143 00:10:59,760 --> 00:11:04,740 That's just immune system, OK? 144 00:11:04,740 --> 00:11:07,350 Another abbreviation that I'll use 145 00:11:07,350 --> 00:11:11,220 is when I refer to an antibody, I'm going to abbreviate it, AB. 146 00:11:20,630 --> 00:11:24,130 All right, so we have these two branches of the immune system, 147 00:11:24,130 --> 00:11:27,250 and they each have the type of antigen receptor, 148 00:11:27,250 --> 00:11:30,580 so now I want to go through what these different types 149 00:11:30,580 --> 00:11:33,750 of antigen receptors look like, OK? 150 00:11:33,750 --> 00:11:44,770 And I'm going to start with the B-cell antigen receptor, also 151 00:11:44,770 --> 00:11:53,460 known as the antibody, also known as an immunoglobulin. 152 00:11:53,460 --> 00:11:58,450 OK, so another-- these are all synonymous, 153 00:11:58,450 --> 00:12:01,420 but you will see them in different contexts. 154 00:12:01,420 --> 00:12:04,220 Immunoglobulin is abbreviated IG. 155 00:12:08,430 --> 00:12:14,170 And what the antibody looks like structurally, 156 00:12:14,170 --> 00:12:17,250 it looks like this, and I'll just 157 00:12:17,250 --> 00:12:19,950 draw it out for you down here. 158 00:12:19,950 --> 00:12:22,080 So I'm drawing a lipid bilayer that 159 00:12:22,080 --> 00:12:24,430 represents the plasma membrane. 160 00:12:24,430 --> 00:12:26,730 The outside of the cell is going to be up, 161 00:12:26,730 --> 00:12:29,140 so that's the exoplasm up here. 162 00:12:29,140 --> 00:12:31,580 The inside down here is the cytoplasm. 163 00:12:34,600 --> 00:12:38,230 And this would be a B cell, then, we're talking about here. 164 00:12:38,230 --> 00:12:43,800 I'm going to draw just a segment of the B cell plasma membrane. 165 00:12:43,800 --> 00:12:48,540 And the B-- the antibody can have a transmembrane domain 166 00:12:48,540 --> 00:12:53,220 that spans the plasma membrane, and then there are domains-- 167 00:12:56,640 --> 00:13:01,050 and what I'm drawing here is a circle, is an IG domain, 168 00:13:01,050 --> 00:13:03,435 so this is going to equal an IG domain. 169 00:13:07,460 --> 00:13:12,510 It's just a type of protein fold that is modular, OK? 170 00:13:12,510 --> 00:13:15,900 So you can see up on my diagram here, right? 171 00:13:15,900 --> 00:13:20,520 You see these like here there are these two green segments 172 00:13:20,520 --> 00:13:24,810 labeled V and C. Each of those is a single IG domain. 173 00:13:24,810 --> 00:13:27,870 OK, it's just a modular fold that 174 00:13:27,870 --> 00:13:30,720 is separate from the other part of the protein. 175 00:13:33,400 --> 00:13:40,740 OK, so here we have along-- 176 00:13:40,740 --> 00:13:43,500 this is one polypeptide chain that 177 00:13:43,500 --> 00:13:47,650 has a transmembrane domain, and it is inserted into the plasma 178 00:13:47,650 --> 00:13:48,150 membrane. 179 00:13:48,150 --> 00:13:52,800 The N-terminus is here, the C-terminus is down here, 180 00:13:52,800 --> 00:14:01,440 and each antibody protein has two of these long peptides. 181 00:14:01,440 --> 00:14:04,470 And because they're the longest part of the molecule, 182 00:14:04,470 --> 00:14:07,410 they're known as heavy chains, so these are the heavy chains. 183 00:14:10,530 --> 00:14:13,500 And each antibody protein is composed of two 184 00:14:13,500 --> 00:14:15,840 identical heavy chains, OK? 185 00:14:15,840 --> 00:14:18,070 So these are identical. 186 00:14:18,070 --> 00:14:20,550 And then also there's another component, 187 00:14:20,550 --> 00:14:27,090 which is present up here, and this is a shorter polypeptide. 188 00:14:27,090 --> 00:14:29,670 And because it's shorter and smaller, 189 00:14:29,670 --> 00:14:31,460 it's known as the light chain. 190 00:14:31,460 --> 00:14:32,760 OK, that's the light chain. 191 00:14:38,320 --> 00:14:42,310 OK, so that's more or less what an antibody looks like. 192 00:14:42,310 --> 00:14:45,400 The part of this antigen receptor that 193 00:14:45,400 --> 00:14:49,430 recognizes the antigen are the tips right here, 194 00:14:49,430 --> 00:14:55,030 so this is where the antigen binds, 195 00:14:55,030 --> 00:14:58,180 and it can bind on either this side or this side. 196 00:14:58,180 --> 00:15:01,570 This molecule is laterally symmetric. 197 00:15:01,570 --> 00:15:03,880 One side is identical to the other, OK? 198 00:15:06,940 --> 00:15:15,830 Now, the T-cell receptor looks different, 199 00:15:15,830 --> 00:15:18,790 and the T cell receptor has fewer names. 200 00:15:18,790 --> 00:15:22,030 It's just called the T-cell receptor, or the TCR, 201 00:15:22,030 --> 00:15:23,770 for short. 202 00:15:23,770 --> 00:15:28,450 And the T-cell receptor is structurally very different, 203 00:15:28,450 --> 00:15:32,860 so now I'm drawing here a T-cell plasma membrane. 204 00:15:32,860 --> 00:15:34,600 Here's the plasma membrane. 205 00:15:34,600 --> 00:15:36,970 The exoplasm, again, is up. 206 00:15:36,970 --> 00:15:42,220 The cytoplasm is down below this plasma membrane. 207 00:15:42,220 --> 00:15:47,380 And the T-cell receptor has two chains. 208 00:15:47,380 --> 00:15:51,970 One is called alpha and the other is beta, 209 00:15:51,970 --> 00:15:55,660 and it has fewer immunoglobulin repeats, 210 00:15:55,660 --> 00:16:00,730 so that you can see you just have this sort of smaller 211 00:16:00,730 --> 00:16:04,090 system here, where you have an alpha and a beta chain. 212 00:16:04,090 --> 00:16:06,760 And in this case, this region here 213 00:16:06,760 --> 00:16:08,990 recognizes the antigen, OK? 214 00:16:08,990 --> 00:16:12,280 So basically the T-cell receptor, or the tip of it, 215 00:16:12,280 --> 00:16:15,880 interacts with the antigen. 216 00:16:15,880 --> 00:16:20,320 Now, the B-cell receptor, or the antibody, 217 00:16:20,320 --> 00:16:24,910 has different forms, so let's talk about the different forms. 218 00:16:27,790 --> 00:16:31,060 And these are shown up on my slide above, right? 219 00:16:31,060 --> 00:16:34,690 So you see over here, here is an antibody that 220 00:16:34,690 --> 00:16:37,720 has a transmembrane domain and is anchored in the plasma 221 00:16:37,720 --> 00:16:40,510 membrane, but there's another form that 222 00:16:40,510 --> 00:16:43,660 lacks that transmembrane domain, and instead 223 00:16:43,660 --> 00:16:46,660 of being an integral membrane protein, 224 00:16:46,660 --> 00:16:51,070 is instead secreted into the blood, OK? 225 00:16:51,070 --> 00:16:53,770 So the forms of the B cell receptor 226 00:16:53,770 --> 00:16:59,380 are both a membrane-bound form, which is initially 227 00:16:59,380 --> 00:17:03,670 how this antibody is presented, but later on, it 228 00:17:03,670 --> 00:17:06,790 can be secreted, and this often changes 229 00:17:06,790 --> 00:17:10,480 when there is an infection, OK? 230 00:17:10,480 --> 00:17:14,810 So once you have a virus or bacteria in your system, 231 00:17:14,810 --> 00:17:16,750 then you get the B cells sort of pumping out 232 00:17:16,750 --> 00:17:18,550 the secreted form of the antibody 233 00:17:18,550 --> 00:17:21,220 in order to fight the infection, OK? 234 00:17:21,220 --> 00:17:25,780 In contrast, for the T-cell receptor. 235 00:17:25,780 --> 00:17:28,720 For the T-cell receptor, there's only one form, which 236 00:17:28,720 --> 00:17:31,210 is the membrane-bound form, OK? 237 00:17:31,210 --> 00:17:33,910 So for T-cell receptors, it's membrane only. 238 00:17:39,040 --> 00:17:42,370 OK, another thing that differs between these antigen centers 239 00:17:42,370 --> 00:17:47,290 receptors is the types of antigens that are recognized. 240 00:17:47,290 --> 00:17:52,020 So antibodies can recognize all sorts of different molecules, 241 00:17:52,020 --> 00:17:52,520 OK? 242 00:17:52,520 --> 00:17:56,290 They're very promiscuous, but they-- 243 00:17:56,290 --> 00:17:59,180 and a given antibody is not promiscuous. 244 00:17:59,180 --> 00:18:04,030 A given antibody will recognize a very specific structure, 245 00:18:04,030 --> 00:18:06,340 but the possibility for antibodies 246 00:18:06,340 --> 00:18:09,910 is that they can recognize small molecules. 247 00:18:12,730 --> 00:18:19,330 They can recognize proteins, they can recognize DNA, 248 00:18:19,330 --> 00:18:21,820 they can recognize carbohydrates, 249 00:18:21,820 --> 00:18:23,560 you get the idea, right? 250 00:18:23,560 --> 00:18:28,420 They really can recognize a whole range of different types 251 00:18:28,420 --> 00:18:29,440 of molecules. 252 00:18:32,060 --> 00:18:37,550 In contrast, the T-cell receptor is more restricted 253 00:18:37,550 --> 00:18:42,110 in that T-cell receptors will recognize peptides or short 254 00:18:42,110 --> 00:18:44,850 sequences of amino acids. 255 00:18:44,850 --> 00:18:50,240 So it recognizes peptides, and these peptides 256 00:18:50,240 --> 00:18:59,480 are presented to the T cell on a type of molecule 257 00:18:59,480 --> 00:19:04,400 presented by the MHC complex. 258 00:19:04,400 --> 00:19:08,210 There are two classes, 1 and 2, and we're 259 00:19:08,210 --> 00:19:11,660 going to talk about this in detail in Friday's lecture. 260 00:19:11,660 --> 00:19:14,330 So I just want to point out the difference 261 00:19:14,330 --> 00:19:18,080 in the types of antigens that can be recognized here, 262 00:19:18,080 --> 00:19:22,010 and we'll talk about exactly what that means on Friday. 263 00:19:26,370 --> 00:19:30,490 OK, so now we have to talk about the amazing properties 264 00:19:30,490 --> 00:19:31,940 of the immune system. 265 00:19:31,940 --> 00:19:36,460 The first is how specific it is, its specificity, 266 00:19:36,460 --> 00:19:41,560 and I think this is a really amazing property, the ability 267 00:19:41,560 --> 00:19:48,130 to really discriminate between very closely related molecules, 268 00:19:48,130 --> 00:19:48,940 right? 269 00:19:48,940 --> 00:19:52,450 And this is essential for immunity to work well. 270 00:19:52,450 --> 00:19:57,040 You want to recognize things that our foreign agents that 271 00:19:57,040 --> 00:19:59,500 have like invaded your system. 272 00:19:59,500 --> 00:20:04,330 You don't want to be recognizing proteins and structures that 273 00:20:04,330 --> 00:20:06,550 are natively present in your body, 274 00:20:06,550 --> 00:20:08,890 because if your immune system did that, you'd 275 00:20:08,890 --> 00:20:12,730 have an autoimmune disease, so this specificity 276 00:20:12,730 --> 00:20:17,180 is really crucial for the function of the immune system. 277 00:20:17,180 --> 00:20:20,770 So now I want to talk how is it that the immune system achieves 278 00:20:20,770 --> 00:20:25,390 such high levels of specificity, and the way 279 00:20:25,390 --> 00:20:28,990 I want to illustrate this is I want 280 00:20:28,990 --> 00:20:32,810 to bring this down quickly. 281 00:20:32,810 --> 00:20:36,470 So if we consider the structure of the antibody, 282 00:20:36,470 --> 00:20:43,000 these different domains are different in that-- 283 00:20:43,000 --> 00:20:46,340 in how variable they are, so some are variable. 284 00:20:46,340 --> 00:20:48,700 So this domain here for the heavy chain 285 00:20:48,700 --> 00:20:51,520 is the variable domain of the heavy chain, which I'll just 286 00:20:51,520 --> 00:20:56,260 abbreviate VH, and then these other immunoglobulin domains 287 00:20:56,260 --> 00:20:59,850 are constant, meaning they don't have a lot of variation 288 00:20:59,850 --> 00:21:02,320 in sequence. 289 00:21:02,320 --> 00:21:05,080 Like the heavy chain, there is a variable domain 290 00:21:05,080 --> 00:21:08,740 for the light chain, which I'll abbreviate VL, 291 00:21:08,740 --> 00:21:13,840 and then there is a constant domain for the light chain, OK? 292 00:21:13,840 --> 00:21:17,530 And so what I want to do now is consider 293 00:21:17,530 --> 00:21:21,790 what the sequence variation is here on this antibody 294 00:21:21,790 --> 00:21:23,290 is the same over here. 295 00:21:23,290 --> 00:21:26,450 This is the same thing over here. 296 00:21:26,450 --> 00:21:28,810 You have a variable domain for the heavy chain 297 00:21:28,810 --> 00:21:30,910 and a variable domain for the light chain. 298 00:21:33,680 --> 00:21:39,460 So let's consider the amino acid sequence of the antibody 299 00:21:39,460 --> 00:21:46,220 molecule specifically at that variable part of the protein. 300 00:21:46,220 --> 00:21:50,470 So let's say we could take individual antibodies 301 00:21:50,470 --> 00:21:53,700 and define their sequence from end to C-terminus. 302 00:21:53,700 --> 00:21:58,160 That would be from tip towards the end here. 303 00:21:58,160 --> 00:22:01,960 So if we take a number of different antibodies 304 00:22:01,960 --> 00:22:06,380 and align their amino acid sequence-- 305 00:22:06,380 --> 00:22:07,210 so what I am-- 306 00:22:07,210 --> 00:22:09,640 I'm not writing out an amino acid sequence, 307 00:22:09,640 --> 00:22:12,310 but I'm just illustrating like a particular type 308 00:22:12,310 --> 00:22:15,310 of computational experiment you could do. 309 00:22:15,310 --> 00:22:24,880 So these would be aligned amino acid sequences 310 00:22:24,880 --> 00:22:31,180 where each of these represents a different antibody, let's say, 311 00:22:31,180 --> 00:22:35,680 heavy chain polypeptide that's produced from a different B 312 00:22:35,680 --> 00:22:36,700 cell, OK? 313 00:22:36,700 --> 00:22:45,490 So each of these is a different antibody from a unique B cell. 314 00:22:51,070 --> 00:22:56,260 And then we just consider the residue number 315 00:22:56,260 --> 00:22:58,840 and how much each amino acid residue 316 00:22:58,840 --> 00:23:03,130 varies along this sequence. 317 00:23:03,130 --> 00:23:07,990 So if we were to align antibody gene stretches like this 318 00:23:07,990 --> 00:23:10,480 and look at how much variation there is, 319 00:23:10,480 --> 00:23:13,540 you'd get a graph that looks like this, OK? 320 00:23:13,540 --> 00:23:17,460 So the y-axis is the amount of variation 321 00:23:17,460 --> 00:23:21,700 and the x-axis here is the residue number 322 00:23:21,700 --> 00:23:25,630 along this polypeptide sequence. 323 00:23:25,630 --> 00:23:30,070 And what you see, probably even without the color here, 324 00:23:30,070 --> 00:23:32,830 is that there are these three regions where there's 325 00:23:32,830 --> 00:23:35,110 a lot of variation in the sequence 326 00:23:35,110 --> 00:23:37,240 of different antibodies, OK? 327 00:23:37,240 --> 00:23:40,930 So here you see the blue segment here has a lot of variation, 328 00:23:40,930 --> 00:23:43,330 the yellow segment has a lot of variation, 329 00:23:43,330 --> 00:23:49,870 and the reds segment here has possibly the most variation. 330 00:23:49,870 --> 00:23:59,650 And what these are known as are hypervariable regions, 331 00:23:59,650 --> 00:24:03,080 meaning that they exhibit a lot of variation. 332 00:24:03,080 --> 00:24:05,470 Another name for them is that they 333 00:24:05,470 --> 00:24:08,860 are complementarity-determining regions. 334 00:24:08,860 --> 00:24:10,892 Complementarity. 335 00:24:10,892 --> 00:24:12,655 Complementarity-determining. 336 00:24:20,830 --> 00:24:28,930 Determining regions, or CDRs, and there are 337 00:24:28,930 --> 00:24:33,550 three of them, 1, 2, and 3, OK? 338 00:24:33,550 --> 00:24:37,090 So there are regions in this antibody molecule 339 00:24:37,090 --> 00:24:41,140 which are much more variable than others, OK? 340 00:24:41,140 --> 00:24:43,330 So what are these regions? 341 00:24:43,330 --> 00:24:48,280 Well, this is a sort of crystal structure of the-- 342 00:24:48,280 --> 00:24:51,400 of an antibody, and you can see how the antigen is 343 00:24:51,400 --> 00:24:53,110 bound at the end. 344 00:24:53,110 --> 00:24:55,330 That would be this end of the molecule 345 00:24:55,330 --> 00:24:57,520 or this end of the molecule. 346 00:24:57,520 --> 00:24:59,440 And here you see a ribbon diagram 347 00:24:59,440 --> 00:25:01,300 of the structure of the antibody, 348 00:25:01,300 --> 00:25:03,820 and the complementary complementarity 349 00:25:03,820 --> 00:25:06,790 determining regions are the regions here 350 00:25:06,790 --> 00:25:10,840 that contact the antigen. 351 00:25:10,840 --> 00:25:13,090 And what they are are basically here's 352 00:25:13,090 --> 00:25:17,650 an IG fold, this whole thing, and there are these three loops 353 00:25:17,650 --> 00:25:20,500 that extend out of the end of this molecule, 354 00:25:20,500 --> 00:25:23,650 and you can think of them as three fingers, OK? 355 00:25:23,650 --> 00:25:27,280 Then these fingers are able to reach out and sort of grab on 356 00:25:27,280 --> 00:25:31,120 to like a foreign particle and/or any particle 357 00:25:31,120 --> 00:25:34,630 and stick to it, OK? 358 00:25:34,630 --> 00:25:38,740 So these are the variable regions, 359 00:25:38,740 --> 00:25:42,040 and they have differences in amino acids-- 360 00:25:42,040 --> 00:25:45,460 in amino acid sequence, and even very small 361 00:25:45,460 --> 00:25:56,230 differences in the amino acid sequence 362 00:25:56,230 --> 00:26:00,010 at this particular part of the antibody 363 00:26:00,010 --> 00:26:02,770 can have a huge effect on whether or not 364 00:26:02,770 --> 00:26:04,660 they're able to stick to something, right? 365 00:26:04,660 --> 00:26:06,670 You can imagine if I lost my thumb, 366 00:26:06,670 --> 00:26:09,220 then right now, I'm not able to sort of stick 367 00:26:09,220 --> 00:26:11,290 to that anymore, OK? 368 00:26:11,290 --> 00:26:14,860 So small differences in amino acid sequence 369 00:26:14,860 --> 00:26:24,610 result in large changes in the affinity of this antibody 370 00:26:24,610 --> 00:26:31,630 for an antigen. And antibodies have different sequences, 371 00:26:31,630 --> 00:26:36,750 meaning that they're able to bind to specific substances 372 00:26:36,750 --> 00:26:38,490 differently. 373 00:26:38,490 --> 00:26:40,980 So if an antibody has one set of sequence, 374 00:26:40,980 --> 00:26:43,260 it might recognize one structure. 375 00:26:43,260 --> 00:26:45,033 If it has another sequence, it might 376 00:26:45,033 --> 00:26:46,200 recognize another structure. 377 00:26:49,170 --> 00:26:52,290 So just by changing the sequence at 378 00:26:52,290 --> 00:26:54,090 these complementarity-determining 379 00:26:54,090 --> 00:26:57,630 regions has a huge influence on what these proteins will 380 00:26:57,630 --> 00:27:01,080 bind to, OK? 381 00:27:01,080 --> 00:27:05,340 Now, each B cell expresses a unique antibody 382 00:27:05,340 --> 00:27:08,700 and just one unique antibody. 383 00:27:08,700 --> 00:27:18,000 So each B cell in our body expresses one and only 384 00:27:18,000 --> 00:27:23,100 one antibody protein, and that antibody protein 385 00:27:23,100 --> 00:27:29,630 has a unique sequence at the CDR region, 386 00:27:29,630 --> 00:27:44,450 and this one antibody has unique specificity for an antigen, OK? 387 00:27:44,450 --> 00:27:46,970 So here you can see in my diagram, 388 00:27:46,970 --> 00:27:49,520 I have a whole bunch of B cells here. 389 00:27:49,520 --> 00:27:53,300 They all express a different antibody, 390 00:27:53,300 --> 00:27:56,030 and you can see that the way you could get more of a given 391 00:27:56,030 --> 00:28:01,310 antibody is to clonally expand one of these cells, 392 00:28:01,310 --> 00:28:05,240 and all of the cells that result from that colonial expansion 393 00:28:05,240 --> 00:28:07,670 will express the exact same antibody. 394 00:28:11,330 --> 00:28:15,230 And when you have a clonal population of a cell 395 00:28:15,230 --> 00:28:23,750 that all has the same antibody, that's known as monoclonal, OK? 396 00:28:23,750 --> 00:28:28,580 So each B cell will have a B-cell receptor or an antibody 397 00:28:28,580 --> 00:28:30,930 with unique specificity. 398 00:28:30,930 --> 00:28:35,120 So now the question becomes, OK, so I told you 399 00:28:35,120 --> 00:28:37,490 how you get specificity, but in order 400 00:28:37,490 --> 00:28:39,620 to have a functioning immune system, 401 00:28:39,620 --> 00:28:42,320 you need to have lots of different cells 402 00:28:42,320 --> 00:28:47,210 that each express a different cell receptor, 403 00:28:47,210 --> 00:28:50,480 so there needs to be a way to generate diversity. 404 00:28:54,800 --> 00:28:57,440 And the answer to how we generate diversity 405 00:28:57,440 --> 00:29:00,110 has an MIT connection. 406 00:29:00,110 --> 00:29:04,910 The research wasn't done at MIT, but the person 407 00:29:04,910 --> 00:29:08,810 who discovered the mechanism is now at MIT. 408 00:29:08,810 --> 00:29:16,190 This research was performed by Susumu Tonegawa, 409 00:29:16,190 --> 00:29:19,460 and Professor Tonegawa, for his work 410 00:29:19,460 --> 00:29:21,980 on how this diversity is generated, 411 00:29:21,980 --> 00:29:29,640 was awarded the Nobel Prize in medicine in 1987. 412 00:29:29,640 --> 00:29:33,390 OK, so Professor Tonegawa did this research elsewhere, 413 00:29:33,390 --> 00:29:36,230 but now he is a faculty member here at MIT. 414 00:29:41,420 --> 00:29:42,860 All right, so diversity. 415 00:29:42,860 --> 00:29:44,410 The problem of diversity, right? 416 00:29:44,410 --> 00:29:48,420 We have millions of B cells that have a unique antibody. 417 00:29:48,420 --> 00:29:50,950 OK, so one solution to this problem 418 00:29:50,950 --> 00:29:54,830 would be we have a million different antibody genes, 419 00:29:54,830 --> 00:29:58,180 and each B cell clone sort of expresses one of them 420 00:29:58,180 --> 00:30:01,540 OK how many genes do we have? 421 00:30:01,540 --> 00:30:05,495 Anyone know, roughly, on the order of magnitude? 422 00:30:05,495 --> 00:30:06,370 Do we have a million? 423 00:30:06,370 --> 00:30:07,142 What's that? 424 00:30:07,142 --> 00:30:08,350 AUDIENCE: 30,000? 425 00:30:08,350 --> 00:30:10,200 ADAM MARTIN: Exactly. 426 00:30:10,200 --> 00:30:12,850 Yeah, so Mr. George has suggested-- 427 00:30:12,850 --> 00:30:14,560 Miles, I believe. 428 00:30:14,560 --> 00:30:15,460 Yeah, OK, good. 429 00:30:15,460 --> 00:30:20,170 Miles suggested 30,000, which is the good upper limit, right? 430 00:30:20,170 --> 00:30:23,080 So having a million antibody genes 431 00:30:23,080 --> 00:30:27,490 sounds a little bit unfeasible, OK? 432 00:30:27,490 --> 00:30:30,490 And so it's basically unfeasible for us 433 00:30:30,490 --> 00:30:33,760 to express as many antibody genes 434 00:30:33,760 --> 00:30:37,330 or have as many antibody genes as we have antibodies. 435 00:30:37,330 --> 00:30:41,350 We just don't have enough real estate in our genome, OK? 436 00:30:41,350 --> 00:30:43,840 But there's another solution to generate 437 00:30:43,840 --> 00:30:46,510 the diversity, which is essentially 438 00:30:46,510 --> 00:30:48,760 a form of shuffling. 439 00:30:48,760 --> 00:30:55,430 So we have a single heavy chain gene for antibodies, 440 00:30:55,430 --> 00:30:58,060 and we have two genes for the light chain, 441 00:30:58,060 --> 00:31:04,090 but these genes are composed of multiple gene segments. 442 00:31:04,090 --> 00:31:06,340 There are multiple gene segments. 443 00:31:09,580 --> 00:31:12,250 Specifically, the segments that make up-- 444 00:31:15,120 --> 00:31:18,990 that generate this variable domain is composed 445 00:31:18,990 --> 00:31:23,310 of multiple gene segments, and these gene-shaped segments 446 00:31:23,310 --> 00:31:27,030 are shuffled during the development of the B cell 447 00:31:27,030 --> 00:31:29,700 to give rise to different proteins. 448 00:31:29,700 --> 00:31:36,390 OK, so these gene segments are shuffled to generate 449 00:31:36,390 --> 00:31:37,380 this diversity. 450 00:31:43,790 --> 00:31:47,550 OK, so now I'm showing you on the top here, 451 00:31:47,550 --> 00:31:53,300 this is the human immunoglobulin heavy chain locus here. 452 00:31:53,300 --> 00:31:54,600 You can see it's pretty big. 453 00:31:54,600 --> 00:31:56,310 There are lots of components. 454 00:31:56,310 --> 00:31:59,070 I want you to focus on this. 455 00:31:59,070 --> 00:32:04,020 So there is-- you see in orange, there's this variable gene 456 00:32:04,020 --> 00:32:08,820 segment, and there are 45 variable gene segments here. 457 00:32:08,820 --> 00:32:11,190 There's this diversity, or D segment 458 00:32:11,190 --> 00:32:15,360 here, which there are 23 of, and then there 459 00:32:15,360 --> 00:32:20,480 are six of these joining or J segments. 460 00:32:20,480 --> 00:32:25,020 OK, so these are all distinct parts of the gene. 461 00:32:25,020 --> 00:32:27,600 They're all distinct parts of the exon 462 00:32:27,600 --> 00:32:31,360 that encodes this variable region of the antibody, OK? 463 00:32:34,680 --> 00:32:44,835 So you have multiple V, D, and J gene segments. 464 00:32:49,410 --> 00:32:53,220 And in order to generate a functional antibody, 465 00:32:53,220 --> 00:32:56,820 one V has to be brought together with one D, which 466 00:32:56,820 --> 00:33:02,970 has to be brought together with one J for that heavy chain, OK? 467 00:33:02,970 --> 00:33:06,600 So you have multiple V-D gene segments, 468 00:33:06,600 --> 00:33:14,700 and they have to be brought together 469 00:33:14,700 --> 00:33:18,767 to form a functional antibody. 470 00:33:22,510 --> 00:33:25,540 OK, that's illustrated right here. 471 00:33:25,540 --> 00:33:27,390 So here you see this is the light chain. 472 00:33:27,390 --> 00:33:30,630 For the light chain, there are only V and J gene segments. 473 00:33:30,630 --> 00:33:35,310 V For the heavy chain, there there's V, D, and J. 474 00:33:35,310 --> 00:33:39,040 And so most of the cells in our body 475 00:33:39,040 --> 00:33:42,390 and the cells of our germline, at the very earliest 476 00:33:42,390 --> 00:33:45,460 stages of development, all have this arrangement, 477 00:33:45,460 --> 00:33:48,270 where you have everything still intact. 478 00:33:48,270 --> 00:33:50,130 But during lymphocytes development, 479 00:33:50,130 --> 00:33:54,960 specifically in lymphocytes, there is a recombination event 480 00:33:54,960 --> 00:33:58,675 that brings together V and J segments or V, D, 481 00:33:58,675 --> 00:34:01,740 and J segments, OK? 482 00:34:01,740 --> 00:34:09,420 So this is mediated by recombination 483 00:34:09,420 --> 00:34:23,219 at the heavy and light chain genes for that antibody, OK? 484 00:34:23,219 --> 00:34:26,010 And so this is very different from the recombination we 485 00:34:26,010 --> 00:34:29,520 talked about earlier in the semester, where recombination 486 00:34:29,520 --> 00:34:33,159 is happening during meiosis and the formation of the gametes, 487 00:34:33,159 --> 00:34:33,659 right? 488 00:34:33,659 --> 00:34:36,239 In that case, recombination is happening 489 00:34:36,239 --> 00:34:38,550 between homologous chromosomes. 490 00:34:38,550 --> 00:34:41,280 Here we're not talking about recombination between 491 00:34:41,280 --> 00:34:43,330 homologous chromosomes. 492 00:34:43,330 --> 00:34:45,239 We're talking about recombination 493 00:34:45,239 --> 00:34:49,170 that brings together and deletes segments 494 00:34:49,170 --> 00:34:52,650 along a single chromosome to bring these V and J 495 00:34:52,650 --> 00:34:54,600 segments together, OK? 496 00:34:54,600 --> 00:34:56,820 So this is sort of a intra-chromosomal 497 00:34:56,820 --> 00:35:01,350 recombination, which deletes the intervening sequences 498 00:35:01,350 --> 00:35:05,280 and brings these gene segments together to form a functional 499 00:35:05,280 --> 00:35:07,540 antibody protein. 500 00:35:07,540 --> 00:35:17,040 So this process is known as V(D)J recombination, 501 00:35:17,040 --> 00:35:18,765 and this is lymphocyte specific. 502 00:35:25,310 --> 00:35:29,710 OK, and that's because during the development of B and T 503 00:35:29,710 --> 00:35:36,370 cells, there is an induction of recombinases that 504 00:35:36,370 --> 00:35:39,260 mediate this recombination. 505 00:35:39,260 --> 00:35:45,830 So in this case, there is recombination, 506 00:35:45,830 --> 00:35:50,890 which is mediated by recombination-activating genes 507 00:35:50,890 --> 00:35:55,660 1 and 2, called RAG1 and 2, OK? 508 00:35:55,660 --> 00:35:58,030 So there are these are lymphocyte-specific 509 00:35:58,030 --> 00:36:02,050 recombinases which mediate this rearrangement, which 510 00:36:02,050 --> 00:36:07,270 bring together a unique V, D, and J segments together, OK? 511 00:36:07,270 --> 00:36:09,820 So the diversity comes from the fact 512 00:36:09,820 --> 00:36:21,625 that each of these V, D and J segments, each V segment-- 513 00:36:24,460 --> 00:36:27,730 you could-- this also applies to D segments and also J 514 00:36:27,730 --> 00:36:28,750 segments-- 515 00:36:28,750 --> 00:36:29,710 has a unique sequence. 516 00:36:35,050 --> 00:36:38,740 So it encodes for a unique amino acid sequence, 517 00:36:38,740 --> 00:36:40,720 meaning that if you bring together 518 00:36:40,720 --> 00:36:44,260 different combinations of Vs, Ds, and Js, 519 00:36:44,260 --> 00:36:47,110 you get a distinct protein, OK? 520 00:36:47,110 --> 00:36:51,760 Now even if you had all of the combinations of V, Ds, and Js, 521 00:36:51,760 --> 00:36:53,650 you still don't have the diversity 522 00:36:53,650 --> 00:36:55,570 that we see in the human body. 523 00:36:55,570 --> 00:36:59,900 So there is another process that further generates diversity, 524 00:36:59,900 --> 00:37:02,260 which is the fact that when these segments are getting 525 00:37:02,260 --> 00:37:07,570 shuffled, it's imprecise in that nucleotides can be inserted 526 00:37:07,570 --> 00:37:11,000 or deleted as these segments are joined, 527 00:37:11,000 --> 00:37:14,402 which generates greater amino acid diversity, 528 00:37:14,402 --> 00:37:15,235 and this is called-- 529 00:37:19,720 --> 00:37:22,123 it's called junctional imprecision. 530 00:37:26,860 --> 00:37:29,960 So this recombination is not precise, 531 00:37:29,960 --> 00:37:35,780 but it leads to the insertion or deletion 532 00:37:35,780 --> 00:37:41,420 of nucleotides of nucleotides. 533 00:37:47,150 --> 00:37:52,220 And if there's a multiple of 3 nucleotides 534 00:37:52,220 --> 00:37:57,380 either inserted or deleted, then you get a functional antibody. 535 00:37:57,380 --> 00:38:01,260 Why is it that it has to be a multiple of 3? 536 00:38:01,260 --> 00:38:01,760 Jeremy? 537 00:38:01,760 --> 00:38:03,880 AUDIENCE: Otherwise, you end up with a frameshift mutation. 538 00:38:03,880 --> 00:38:04,480 ADAM MARTIN: Exactly. 539 00:38:04,480 --> 00:38:04,980 Right? 540 00:38:04,980 --> 00:38:06,250 This is and the-- 541 00:38:06,250 --> 00:38:09,430 this is on the more sort of like on the N-terminus 542 00:38:09,430 --> 00:38:11,170 side of the gene, right? 543 00:38:11,170 --> 00:38:16,030 So if you inserted one nucleotide between V and J, 544 00:38:16,030 --> 00:38:19,420 then the downstream portion of the gene, the downstream part 545 00:38:19,420 --> 00:38:22,260 of the open reading frame would be out of frame 546 00:38:22,260 --> 00:38:24,630 and wouldn't generate a functional protein. 547 00:38:24,630 --> 00:38:27,340 OK, so it has to be a multiple of 3. 548 00:38:30,140 --> 00:38:30,800 Yeah, Georgia? 549 00:38:30,800 --> 00:38:34,010 AUDIENCE: How is functional precision lymphocyte-specific? 550 00:38:34,010 --> 00:38:35,260 Or is it not? 551 00:38:35,260 --> 00:38:37,550 ADAM MARTIN: It's just the RAG1 and RAG2 552 00:38:37,550 --> 00:38:42,239 are turned on specifically in the lymphocytes as they mature. 553 00:38:42,239 --> 00:38:45,140 AUDIENCE: And that also affects the insertion, deletion? 554 00:38:45,140 --> 00:38:47,265 ADAM MARTIN: Well, if you don't have recombination, 555 00:38:47,265 --> 00:38:49,580 you can't get junctional precision, right? 556 00:38:49,580 --> 00:38:52,580 So the junctional imprecision-- or junctional imprecision. 557 00:38:52,580 --> 00:38:55,610 The junctional imprecision is a consequence 558 00:38:55,610 --> 00:38:59,000 of the recombination process itself, right? 559 00:38:59,000 --> 00:39:01,100 So if you're not having recombination, 560 00:39:01,100 --> 00:39:03,350 you're not having any junctional imprecision 561 00:39:03,350 --> 00:39:06,750 because you're not generating a junction. 562 00:39:06,750 --> 00:39:10,130 OK, now there's one more thing that's important here, 563 00:39:10,130 --> 00:39:13,820 which is something that happens not 564 00:39:13,820 --> 00:39:17,220 as a consequence of this recombination process 565 00:39:17,220 --> 00:39:20,390 but as a consequence of activating the T cell 566 00:39:20,390 --> 00:39:24,950 response, which is that in addition to these variations, 567 00:39:24,950 --> 00:39:29,240 there's also something known as somatic mutation. 568 00:39:34,340 --> 00:39:40,400 So there's an elevated mutation rate at the IG locus 569 00:39:40,400 --> 00:39:44,270 that further increases the diversity of the amino acid 570 00:39:44,270 --> 00:39:49,250 sequence at these variable regions of the antibody, OK? 571 00:39:49,250 --> 00:39:52,130 Another way this is referred to is 572 00:39:52,130 --> 00:39:55,580 because it can increase the affinity of the antibody 573 00:39:55,580 --> 00:40:02,090 for a antigen, it's also known as affinity maturation, 574 00:40:02,090 --> 00:40:05,540 so these are synonymous. 575 00:40:05,540 --> 00:40:08,120 Maturation. 576 00:40:08,120 --> 00:40:10,770 Maturation. 577 00:40:10,770 --> 00:40:14,390 OK, so-- and this depends on the T 578 00:40:14,390 --> 00:40:18,870 cell, the cell-mediated branch of adapted immunity, 579 00:40:18,870 --> 00:40:20,105 so this is T-cell mediated. 580 00:40:30,890 --> 00:40:34,370 So one other aspect of this process 581 00:40:34,370 --> 00:40:39,320 that I want to talk about is until this recombination 582 00:40:39,320 --> 00:40:43,700 happens, the immunoglobulin gene is not expressed, 583 00:40:43,700 --> 00:40:46,820 so it's this recombination that leads 584 00:40:46,820 --> 00:40:50,960 to the expression of the-- either the heavy chain 585 00:40:50,960 --> 00:40:53,660 or the light chain gene, OK? 586 00:40:53,660 --> 00:40:57,770 And that's because the enhancer is sort of downstream 587 00:40:57,770 --> 00:41:01,440 in the gene, and by deleting the intervening sequence here, 588 00:41:01,440 --> 00:41:04,220 you bring the promoter in range of the enhancer, 589 00:41:04,220 --> 00:41:07,880 and now this gene is expressed, OK? 590 00:41:07,880 --> 00:41:10,980 But remember you have two copies of each of these genes. 591 00:41:10,980 --> 00:41:13,640 You have a parental copy and a maternal copy, 592 00:41:13,640 --> 00:41:16,190 and another feature of this system 593 00:41:16,190 --> 00:41:19,070 is that there is what is known as allelic exclusion. 594 00:41:24,380 --> 00:41:29,900 So the system is such that a B cell expresses only one 595 00:41:29,900 --> 00:41:33,830 antibody, and so if you had both alleles expressing, 596 00:41:33,830 --> 00:41:35,900 that wouldn't be the case, OK? 597 00:41:35,900 --> 00:41:40,700 So allelic exclusion makes it that if you get a recombination 598 00:41:40,700 --> 00:41:44,960 event that leads to a functional antibody for one 599 00:41:44,960 --> 00:41:49,850 of your sort of inherited copies of the gene, one 600 00:41:49,850 --> 00:41:53,850 of your alleles, it suppresses recombination on the other one, 601 00:41:53,850 --> 00:41:54,350 OK? 602 00:41:54,350 --> 00:41:58,460 So you will only get one of these genes, one heavy chain 603 00:41:58,460 --> 00:42:01,370 and one light chain, expressed per B cell. 604 00:42:01,370 --> 00:42:12,260 OK, so only one gene expressed so that each B cell only 605 00:42:12,260 --> 00:42:13,850 has one antibody. 606 00:42:19,690 --> 00:42:23,290 OK, I just wanted to point out, finally, 607 00:42:23,290 --> 00:42:28,270 that these junctions between V-D and J segments 608 00:42:28,270 --> 00:42:31,600 fall right in this CDR-3 region, so they're 609 00:42:31,600 --> 00:42:34,870 responsible for the high level of variability 610 00:42:34,870 --> 00:42:40,840 at the CDR or hypervariable 3 region. 611 00:42:40,840 --> 00:42:43,690 OK, and because of the allelic exclusion, 612 00:42:43,690 --> 00:42:48,700 each B cell expresses only one antibody, OK? 613 00:42:48,700 --> 00:42:51,730 So all of the antibody proteins expressed by that cell 614 00:42:51,730 --> 00:42:55,060 will be exactly the same. 615 00:42:55,060 --> 00:42:59,620 OK, so now the last property of the immune system we 616 00:42:59,620 --> 00:43:00,970 need to talk about is memory. 617 00:43:06,970 --> 00:43:10,120 And so the immune system needs to be 618 00:43:10,120 --> 00:43:16,450 able to recall past infectious agents that it's experienced, 619 00:43:16,450 --> 00:43:18,010 and so it needs-- 620 00:43:18,010 --> 00:43:20,050 I guess we're kind of personifying here, 621 00:43:20,050 --> 00:43:22,090 but it needs some sort of memory, right? 622 00:43:22,090 --> 00:43:26,795 It needs the ability to recall this, OK? 623 00:43:26,795 --> 00:43:29,620 And this is the principle behind vaccination, right? 624 00:43:40,190 --> 00:43:43,340 The way vaccines work is to put in one 625 00:43:43,340 --> 00:43:47,270 of these attenuated or inactivated foreign agents, 626 00:43:47,270 --> 00:43:49,610 such that your body is able to remember that 627 00:43:49,610 --> 00:43:51,710 later on when you get the real deal, 628 00:43:51,710 --> 00:43:55,040 and it's able to fight it off, OK? 629 00:43:55,040 --> 00:43:57,800 So the body has to be able to remember. 630 00:43:57,800 --> 00:44:00,920 And several ways in which this manifests itself, 631 00:44:00,920 --> 00:44:04,280 if we compare a primary infection, the first time 632 00:44:04,280 --> 00:44:08,870 you've seen an infectious agent, versus a secondary infection, 633 00:44:08,870 --> 00:44:11,600 they have very different responses 634 00:44:11,600 --> 00:44:17,600 from the standpoint of the adaptive immune system, OK? 635 00:44:17,600 --> 00:44:21,500 So if we consider the lag before your adaptive immune system 636 00:44:21,500 --> 00:44:25,970 really takes off, the primary response 637 00:44:25,970 --> 00:44:30,410 takes about five to 10 days, so it's a bit delayed, 638 00:44:30,410 --> 00:44:34,490 whereas the secondary response can be one to three days, OK? 639 00:44:34,490 --> 00:44:35,700 So it's faster. 640 00:44:35,700 --> 00:44:39,080 It's able to react faster when you see an infectious agent 641 00:44:39,080 --> 00:44:41,090 the second time. 642 00:44:41,090 --> 00:44:45,200 If we also just consider the magnitude of the response 643 00:44:45,200 --> 00:44:47,870 by considering how much antibody, the antibody 644 00:44:47,870 --> 00:44:52,230 concentration that's like put into your system, 645 00:44:52,230 --> 00:44:56,090 then the primary response is smaller 646 00:44:56,090 --> 00:44:59,150 and the magnitude of the secondary response is larger. 647 00:45:02,180 --> 00:45:05,690 So you basically-- your body's able to produce more antibody 648 00:45:05,690 --> 00:45:09,920 against an infectious agent the second time it sees it. 649 00:45:09,920 --> 00:45:14,480 Not only is the antibody amount better the second time, 650 00:45:14,480 --> 00:45:17,090 but actually the antibodies themselves 651 00:45:17,090 --> 00:45:19,610 are better antibodies, OK? 652 00:45:19,610 --> 00:45:26,750 And we can show that by thinking about antibody affinity, which 653 00:45:26,750 --> 00:45:30,560 is how tightly the antibody recognizes the antigen, 654 00:45:30,560 --> 00:45:32,660 and I'll give you numbers that represent 655 00:45:32,660 --> 00:45:38,970 the dissociation constant for an antibody to a given antigen. 656 00:45:38,970 --> 00:45:42,770 So the lower that number is, the tighter the binding. 657 00:45:42,770 --> 00:45:47,030 So for the primary infection, the antibody affinity 658 00:45:47,030 --> 00:45:52,520 is weaker on the order of 10 to the negative 7th molar in terms 659 00:45:52,520 --> 00:45:55,760 of KD, and this secondary infection 660 00:45:55,760 --> 00:45:58,340 generates antibodies that are functionally quite better. 661 00:45:58,340 --> 00:45:59,405 They bind much tighter. 662 00:46:02,210 --> 00:46:06,350 It can be less than 10 to the negative 11th molar, which 663 00:46:06,350 --> 00:46:07,530 is sub-nanomolar. 664 00:46:07,530 --> 00:46:08,030 Right? 665 00:46:08,030 --> 00:46:10,160 That's a really tight interaction 666 00:46:10,160 --> 00:46:12,270 between two molecules. 667 00:46:12,270 --> 00:46:14,300 So the antibodies, you get more of them, 668 00:46:14,300 --> 00:46:16,670 and they're better antibodies, OK? 669 00:46:19,470 --> 00:46:25,860 So what makes this memorable is that when-- what lasts 670 00:46:25,860 --> 00:46:29,700 in your body from the first time you see the agent to the next 671 00:46:29,700 --> 00:46:40,700 is there's a type of B cell known as a memory B cell, 672 00:46:40,700 --> 00:46:46,030 and this memory B cell will express a given antibody, 673 00:46:46,030 --> 00:46:49,760 and that antibody will be specific to the substance 674 00:46:49,760 --> 00:46:51,480 you saw previously. 675 00:46:51,480 --> 00:46:54,350 And because recombination is-- this recombination 676 00:46:54,350 --> 00:46:57,390 is irreversible, then that B cell 677 00:46:57,390 --> 00:47:00,140 is going to remember that antibody because it's still 678 00:47:00,140 --> 00:47:02,240 encoded in the genome. 679 00:47:02,240 --> 00:47:08,480 So the memory results from V(D)J recombination being 680 00:47:08,480 --> 00:47:16,130 irreversible and the fact that these memory B cells stay 681 00:47:16,130 --> 00:47:19,830 in your body, even if the antigen is not present, 682 00:47:19,830 --> 00:47:24,290 so these also stay in the body. 683 00:47:32,010 --> 00:47:35,360 OK, so effective vaccines generate these types 684 00:47:35,360 --> 00:47:37,760 of cells, these memory B cells. 685 00:47:37,760 --> 00:47:40,990 OK, that's important if you want an effective vaccine, 686 00:47:40,990 --> 00:47:43,940 that you have these B cells that retain information 687 00:47:43,940 --> 00:47:47,420 about the past infection. 688 00:47:47,420 --> 00:47:51,980 All right, so what exactly is it that the antibodies do? 689 00:47:51,980 --> 00:47:58,970 So I'll talk about effector functions of antibodies. 690 00:48:01,920 --> 00:48:06,740 So antibodies can bind to a foreign substance 691 00:48:06,740 --> 00:48:09,830 and interfere with the normal function, right? 692 00:48:09,830 --> 00:48:12,560 If you have a bacteria and maybe the antibody 693 00:48:12,560 --> 00:48:15,320 binds to some part of the bacteria 694 00:48:15,320 --> 00:48:18,710 to interfere with that bacteria getting into the cell, 695 00:48:18,710 --> 00:48:22,250 and this type of effect is known as neutralization. 696 00:48:26,870 --> 00:48:29,060 If you had an antibody that bound to something 697 00:48:29,060 --> 00:48:31,700 like a bacteria, you could also have 698 00:48:31,700 --> 00:48:37,350 it recruit phagocytic cells to internalize that bacteria, 699 00:48:37,350 --> 00:48:43,190 and so you could also induce phagocytosis. 700 00:48:43,190 --> 00:48:47,090 In addition, antibodies, when bound to a foreign substance, 701 00:48:47,090 --> 00:48:49,730 if that foreign substance is a cell, 702 00:48:49,730 --> 00:48:54,650 then it could recruit a killing cell to kill that cell, 703 00:48:54,650 --> 00:48:59,750 so there's also a killing aspect to this, OK? 704 00:48:59,750 --> 00:49:03,590 So what's in this diagram here is a type of cell 705 00:49:03,590 --> 00:49:06,920 known as a natural killer cell that is killing its target 706 00:49:06,920 --> 00:49:09,020 cell, and so you can kind of think 707 00:49:09,020 --> 00:49:12,320 of this cell as the Terminator, OK? 708 00:49:12,320 --> 00:49:16,280 So right, if the natural killer cell recognizes this target 709 00:49:16,280 --> 00:49:19,910 here, then it's hasta la vista, baby, 710 00:49:19,910 --> 00:49:23,690 and that cell is dead, OK? 711 00:49:23,690 --> 00:49:27,780 I just want to point out one thing that I mentioned before, 712 00:49:27,780 --> 00:49:32,300 which is that antibodies can be leveraged to generate 713 00:49:32,300 --> 00:49:35,970 treatments for certain types of diseases. 714 00:49:35,970 --> 00:49:39,260 And we talked about a drug called Herceptin-- or not 715 00:49:39,260 --> 00:49:41,570 a drug but a-- it's an antibody, but it's 716 00:49:41,570 --> 00:49:47,310 a treatment for HER2 positive breast cancer, 717 00:49:47,310 --> 00:49:52,670 so this is used to treat HER2-positive breast cancer. 718 00:49:52,670 --> 00:49:58,580 And it's really been a nice success story in the cancer 719 00:49:58,580 --> 00:50:02,300 field because what this-- 720 00:50:02,300 --> 00:50:06,170 what Herceptin is-- it was derived from a mouse antibody, 721 00:50:06,170 --> 00:50:08,840 so this is a mouse monoclonal antibody 722 00:50:08,840 --> 00:50:12,050 that recognizes this HER2 growth factor 723 00:50:12,050 --> 00:50:14,060 receptor, which is over expressed 724 00:50:14,060 --> 00:50:17,970 on 30% of human breast cancers. 725 00:50:17,970 --> 00:50:22,430 And what Herceptin is is that researchers took this mouse 726 00:50:22,430 --> 00:50:26,660 antibody and engineered a human antibody 727 00:50:26,660 --> 00:50:30,800 to have the mouse sequence at its complementarity-determining 728 00:50:30,800 --> 00:50:34,370 regions, such that you have a human antibody that 729 00:50:34,370 --> 00:50:38,990 won't be sort of removed by the human immune system 730 00:50:38,990 --> 00:50:44,150 but will recognize HER2 and recruit human immune cells 731 00:50:44,150 --> 00:50:48,440 to HER2 positive cells, possibly killing those cells 732 00:50:48,440 --> 00:50:52,700 or binding to HER2 and somehow neutralizing the activity 733 00:50:52,700 --> 00:50:55,760 of HER2 on these cancer cells. 734 00:50:55,760 --> 00:50:59,570 So antibodies can be very useful for therapeutics, as well 735 00:50:59,570 --> 00:51:03,990 as being useful in our own bodies to mediate immunity. 736 00:51:03,990 --> 00:51:06,890 OK, we'll talk about T cells on Friday. 737 00:51:06,890 --> 00:51:09,400 Remember to bring your projects.