 Hi, everybody. So tonight we're back to talk about the second half of our immune system conversation, and we're going to look at the acquired immune response. In the last lecture, we did innate immunity, and these were responses to foreign invaders or pathogens or antigens that were very generalized responses. The acquired immune response is very specific. It's very slow. I mean, it takes a while to ramp this thing up. Once we ramp it up, it's not messing around, and we can do some serious damage with the acquired immune response, but it does take us a while to do it. The acquired immune response has memory. It remembers if it has been exposed to a pathogen, a specific pathogen before, and because of that, you often actually don't get a condition twice. Sometimes the pathogen will mutate, and so you think you're getting a cold again and again. This is the same cold. I thought I can't get the same disease twice, but the fact is it's mutated and it's a different cold. So really, every cold you've ever gotten is a different critter that is invading you because you are saving up information about all the pathogens that have infected you and you are ready to respond with the acquired immune response quickly and efficiently if you get infected again. The acquired immune response is mediated by lymphocytes. Now, we had one flavor of lymphocyte in the innate immune response, and that was the natural killer cells, but those guys make up just 2% of all lymphocytes, and everybody else is involved in acquired. All the other lymphocytes are involved in the acquired immune response. So our two main, we actually have two, just like we had two categories of immune system acquired and innate, we have two categories of acquired immunity. We have the humoral immune response and we have the cell mediated immune response, and these are like branches of acquired immunity. They have things in common. Ultimately, I actually have that homeostasis is the last item on our list, but I think I'm going to add one more because I think we're going to have to have one place where we like go through the whole thing and do a big, huge review of the whole process so we can compare and contrast these two systems effectively. So the first thing that we're going to do is we're going to look at three sections, three factors that where we're going to look at are two branches side by side. So we're going to look at the lymphocytes and we're going to compare the lymphocytes that are involved in the humoral immune response and the lymphocytes that are involved in the cell mediated response. Then we're going to look at how the lymphocytes are educated, how they go through their maturation process. They do go to school and not very many of them graduate. So we'll compare and contrast the way that our two types of lymphocytes are sent to school. And then we have to talk about MHC again. In the last lecture, we talked about how it was present on almost all cells except for red blood cells. In this section, we're going to talk about the role that MHC2 carrying cells play in this whole process and then we'll dive in. After we've got all those like general tools, then we'll dive in and look at each branch individually. I'd like to probably do a summary of all of it and then we'll look at how the immune system relates to homeostasis. So it's a big dense amount of content and why don't we get started then? All right.