 MHC2 is exactly the same, it's the same idea but not everybody has it. MHC2, seriously, all right, MHC2 cells, that's so weird. You just have to change color. All right, anyway, MHC2 is only found on cells that are involved in antigen presentation or antigen presenters. And our two antigen presenters, now hold on just a second. Two antigen presenters are B lymphocytes and a kind of white blood cell called a dendritic cell. And its whole job is to present antigens. When they present antigens, this process is required to activate the acquired immune response. So if you can bind, you can find your antigen, awesome, more power to you, but if you don't connect with an antigen presenter, either a B lymphocyte or a dendritic cell that also has your antigen, it's not going to be activated and the acquired immune response will not fly into action. The number of the system are like crazy talk, which just tells you, this stuff's dangerous. This is like dealing with nuclear war. If you are going to push the red button and deliver a nuclear bomb on someone, you better make sure that there aren't any mistakes. There wasn't a typo in the email that made you mad. There wasn't like nobody hacked into that email and sent you something wrong. Like this is an actual threat worthy of nuclear retaliation. All these checks and balances are in place to make sure of that. There's more checks and balances in place and I'm about to tell you how the B's and T's go to school. What you know so far is that all mature B's and T's are out there with their specific receptor, either a T cell receptor or an antibody that's embedded in their cell membrane and they're out looking for their specific antigen. Where is it? Where is it? Even if they bind with their specific antigen, they still have to find an antigen presenter who also has that specific antigen on their MHC2 platform. And if they find it, then they will be activated and we'll talk about what activation means later on. But first, what are the qualities that we need to have in our B cells and T cells? Why bother sending these little rascals to school? Well, I'll tell you right now, if you had to go to school like this, it would be you would not be a happy camper. After you hear about their school, you're going to think that I'm a really easy teacher.