 Let's talk about our lymphocytes. You have B lymphocytes, having adventures with my pen, and you have T lymphocytes. See me smiling? All right, lymphocytes, both. I'm going to draw a little both category here, and I'm going to make my both category purple. Maybe it's my color. Both B cells and T cells, they look the same. They're that big nucleus lymphocyte, kind of a moony looking lymphocyte, not very much cytoplasm. Both of them look that way. Both of them are born in the bone marrow. We already knew that, didn't we? We knew that our lymphocytes were born in the bone marrow because we talked about that when we were looking at blood clotting and at platelets. We knew that. Both go to school, they mature. Let's talk about where they mature. B lymphocytes mature in the bone marrow, and this is where they go to school. Oh, yes, they go to school. In fact, we're going to talk about their school later on in the lecture. B lymphocytes? It's easy to remember actually. B lymphocytes go to school in the bone marrow. We know everybody was born in the bone marrow. T lymphocytes go to school, mature, and go to school in something that starts with a T in the thymus. I think I have a picture of the thymus in case, what do you know? Hey, what happened to me? That ain't cool, man, that ain't cool. Now, the thymus is an endocrine structure. It's an endocrine gland that's found on top of the heart, and it looks like puffy, mushy stuff, so I've never actually seen anyone's thymus before. It'd be interesting to try to find it on our cadavers, but I just wanted you to have a sense of where the thymus actually is. Both, okay, so now we know where they go to school. I'm going to draw a picture of them because they have different kinds of receptors. B cells, B cells are involved in humoral immunity, and really the bottom line is antibodies. It's, we have lots of antibody stuff going on in their humoral immune response. So look, they have very specific little, their receptors are actually antibodies embedded in the cell membrane, and the antibodies, just like you would expect, the antibodies have a very specific, that end, that tip is specific. So this particular B cell is sporting, starting this specific antibody, and that's like its own special receptor. Compare that to the B lymphocytes, I mean the T lymphocytes, because that's what I just said, the T lymphocytes. These guys have a receptor, but it's called a T cell receptor, and it's different. It's just, it's more of a normal receptor that you would imagine, and it has, again, it has that unique special zone here that makes it bind to only certain antigens. So both of these guys, if you like went out and surveyed T cells, all the T cells that you would survey unless they were responding to an immune attack, all the T cells you surveyed would have a different T cell receptor. How's that possible? All the B cells, different antibody, embedded in their cell membranes. That's the specific part. This B cell is only going to respond to this antigen. That was a B cell, did I say B? Nobody knows. This T cell is only going to respond to this antigen, and all of them are that specific. Now, don't you want to know, oh, I want to tell you that this is also called cell-mediated immunity. So cell-mediated immunity is facilitated by T lymphocytes. Humoral immunity is facilitated by B lymphocytes. Before we go on, we need to spend a little bit more time talking about the T lymphocytes, their ultimate checking MHC platforms, MHC1 platforms. Both of them actually have to be activated to platform, and not very many cells have MHC2 platforms. So let's take a second to talk about MHC, and we'll start out with MHC1.