 Okay, the humoral immune response responded to bad guys in the environment outside of a cell. Cell-mediated immunity responds to threats or dangers inside the cell. Now, how's that going to happen? Well, there's one way. The naive T cell, I knew I could get there. Here's my friend. This is a naive T cell. Same thing. Has its little receptor, but the receptor is, has not, we haven't found our antigen yet. And the antigen that it could bind to is going to look something like that, okay? The antigen would fit into this space. So the naive T cell is going to, the first thing that's going to happen is it doesn't come into contact with its own antigen. That's not the first step. The naive T cell must come in contact with its antigen on an antigen-presenting cell like a dendritic cell. So look at this. Here is, I'm going to say antigen presenter and tell me how do cells present antigens on their MHC2 platforms? So a dendritic cell is an example of an antigen-presenting cell. And in this case, you could imagine that this little dendritic cell could be presenting an antigen, oh geez. I'm imagining that my antigen is going to fit into, oh seriously, why does it do that? That's so weird, okay? It's going to fit into this space right here. If, and it's almost like, I can't tell, is this better, is this worse? I can't tell if it's easier or harder to activate your T cells when compared to your B cells. But the antigen presenter has to have come in contact with the specific antigen. And then it has to come in contact with the naive T cell with the specific receptor. And then the naive T cell has to connect so we get activated. Dude, I mean shouldn't I write something down? One, antigen, antigen. Two, antigen is presented on antigen presenter's MHC2 platform. Antigen presenter binds, antigen presenter's MHC2 platform binds to naive T cell receptor. Four, naive T cell gets activated. So there's only four steps here, not five before you get activated. Now, you've been activated. There are two different, multiple different flavors of naive T cells. So depending on what flavor of naive T cell you were, that will mature grown-up T cell, like truly grown-up, like now you can like go out and kill people, that kind of a grown-up. What kind are you going to become? Will some of them become killer T cells? And guess what killer T's do? They go in, they find the antigen on MHC1. What? So they go around and they look for their antigen on their own, how are you doing? I want to see, do you have my antigen? And they go around and they detect every bit, and they check MHC1s. What if they find their antigen? If they find their antigen, yep, apoptosis. Cell suicide, sad story, you're infected, done. If they are a different flavor of activated T cell, they're going to become helper T cells. Do you remember what those guys did? We saw them in the humoral immune response because they were the ones who were antigen presenters that activated our plasma cells, our naive B cells. So our helper T's are necessary to activate our humoral response. And then we also have some memory T's. And wow, I cannot even tell you how much I want to sing memory song. Memory T's function just like the memory B's, they just remember and activate the whole system faster. All right, holy krikenolis. I'm totally satisfied with this whole immune system thing. I mean, really, is that just fantastic? All right, take care of yourself. Be nice to your immune system so your immune system cells can go destroy things.