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Review of B cells, CD4+ T cells and CD8+ T cells. Created by Sal Khan.
Video transcript
Now that we've touched on all of the major players in the specific immune system, what I thought I would do in this video is do a summary so it all fits together a little bit. So the first person or character we got exposed to was the B cell, which I always do in blue. And what made that interesting is that every B cell has its own specific-- or they have membrane bound antibodies, but for each B cell, the membrane bound antibodies on each specific B cell had its own variable portion. So this B cell-- it'll be variable right like that. And if I were to draw another B cell right here, I would draw the variable portion a little bit different. This is why different actual B cells will respond to different antigens or different pathogens that have entered our system. And a B cell gets activated-- let's talk about what happens when it gets activated or what needs to happen. It needs binding of the pathogen onto one of these membrane bound antibodies. But that's not all. I mean, sometimes that's all you need, but usually you also need to be stimulated by a T cell. And you might say, where's the helper T cell stimulate this guy? Well, B cells were also antigen presenting cells so he'll suck this guy in, break him out, and present him on an MHC II complex. Let's say this is an MHC II complex. This guy gets cut up, part of him gets presented right here, and then an activated helper T cell whose variable portion of their T cell receptor is specific to this could come along and activate this character. I'm not drawing that receptor well, but that right there is a helper T cell and that is the B cell. Now, once it's activated, it starts differentiating-- and it starts cloning itself and it can either turn into effector cells-- and this true of B cells or T cells. Once they get activated, they keep cloning or they either turn into effector cells or memory cells. Memory cells stick around a lot longer so that in the future you're going to have many more of this version of B cells. So if you get the same antigen or pathogen in the future, the likelihood of it bumping into this type of B cell is going to be higher so the response will occur faster. The effector B cells produce-- they essentially turn into antibody making machines. So they'll say, gee, this antibody bonds to this antigen that we have in the system now. Let me just produce a ton of them. So it starts building up all of the cellular machinery and it starts producing antibodies like crazy. I want to point out one thing that my wife pointed out to me when I was-- she overheard me making the last video and she's a fellow in hematology and a lot of hematology is immunology. So I definitely have to defer to her. She is the expert on this. In the last video, I kind of very hand-wavingly said, B cells, once they get activated, if they're the effector B cells, they produce antibodies. I want to be very clear. It is only the effector B cells that produce the antibodies and the common term for them-- if someone were to walk up to you and say, what cells in the body are producing antibodies? You wouldn't be wrong if you said effector B cells, but the common term that people expect to hear are plasma cells. Plasma cells and effector B cells are the same thing, but normally when they say, what happens to a B cell when it starts producing antibodies? They then call it a plasma cell. They don't call it a B cell anymore and I want to make that very clear because my wife's like, well, I have attendings that if they asked me, what cell in the body produces antibodies? And if you said, B cell, they would say, no, wrong. It's a plasma cell-- or if you said effector B cell, they wouldn't be happy. They wanted to hear plasma cell. This is the common term used in immunology and apparently rheumatology circles. Did I just say my wife is a hematologist? No, no. She's studying rheumatology. I get confused with all the tologies sometimes. Anyway, that's what the B cells do. And these antibodies can then go attach things and mess up viruses and antigen-- well, viruses or instances of antigens and bacteria-- and tag them for pick up by macrophages or other types of phagocytes. Those were the B cells. Then you have your T cells. And here I'm going to talk about T cells a little bit differently than I had in the last few videos. Just to give a little bit more of a nuance-- so there's two types of T cells. You'll say, hey, they are helper T cells and cytotoxic T cells-- and you're not wrong, but what I'm going to do is do a slightly different differentiation just so that you are familiar with these terms. So there's two types of T cells. All T cells have T cell receptors. But they also have these other proteins on them and some of them have these proteins called-- CD4 proteins and some of them have what's called CD8. So this one right here would be called a CD8 positive T cell. It has the CD8 proteins on it. And this would be called a CD4 positive T cell. I've never used these words before. You're like, gee, where are these coming from? Now, the CD4 receptor is the thing that wants to bind-- this is the thing that helps to go to the MHC II complexes. So most CD4 T cells are helper T cells. Most of the time-- and I want to make it very clear-- immunology is a very-- I mean, this field, people are discovering things on a regular basis. So people are still understanding these things and there's all sorts of special cases, but usually when people talk about CD4 positive T cells, they're talking about helper T cells. So this is normally going to be a helper T cell-- or you could call it T helper, just like that. Likewise, the CD8 proteins, these are attracted to the MHC I complex. This is what brings them to the cells that have the cancer, that have expressed antigens on their MHC I complex. So most of the time CD8 positive T cells are cytotoxic. And oftentimes, before a cell gets activated, they just describe it as a CD4 T cell or a CD8 cell and after it becomes activated and starts wanting to kill things, then maybe you call it cytotoxic. But this is all wordplay. I think you get the general idea. But just to remember what they do, this guy-- we just said he wants to bond to the MHC complex, so you have MHC complex plus presenting some antigen. This is MHC I right here. We learned in the last video, every nucleated cell in the body expresses an MHC I complex. So this is the case where something wacky is happening inside this cell. Maybe a virus has infected it. Maybe it's cancerous. It needs to die, otherwise it's going to keep producing viruses if it's infected by a virus, or otherwise it's going to keep dividing if it's a cancer and infect the rest of the body. So the CD8 kills infected cells. I'll just say bad cells because I don't if you can-- cancer really isn't an infection. Kills bad cells-- cells that are-- if you don't kill them, they're going to keep producing viruses or keep splitting and spreading the cancer, while T cells-- they're attracted to professional antigen presenting cells. And I always do a dendritic cell right here because those are the best antigen presenting cells. And they have MHC II complexes and it's digested some antigen and it presents it right there and then that activates the helper T cell. And then when the helper T cell-- all of these guys, once they're activated, they all go into effector-- they all start differentiating into effector and memory cells. An effector helper T cell does a couple of things. So if we're talking about a helper T cell, it can activate B cells and it also releases cytokines. Let's say this guy gets activated. He'll also start releasing these chemicals, which are really those alarm bells that tell other people to really get in gear-- maybe B cells and cytotoxic T cells start proliferating more rigorously. Actually, part of the cytotoxic T cell activation can be assisted or kind of given a boost by these cytokines-- of so by these alarm bells. So this guy's the alarm ringer, while the CD8 cells or the cytotoxic T cells-- in their effector mode, they kill cells. And of course in the memory mode, there's just a bunch of copies of these originals around that are ready and more than they were originally were, so that in the future if something like this or something like this occurs, they're going to be activated faster because they're going to be bumped into faster. So hopefully that clears up a little bit and I introduced a little bit more tough terminology, but I really want to stress my wife's point because she said, hey, you don't want people out there saying B cells produce antibodies even though it is effector B cells-- activated B cells that have differentiated into effector B cells. Those are what are producing antibodies-- because when they go to medical school, people are going to want to hear plasma cell.