 All right, it's time to go to school, dog-pounds. Time to find yourself. I'm going to draw a picture of my cells that are going to school. Remember that I've got T's that are going to school. Where do they go to school? They go to school in the thymus. And my B's, these are just my lymphocytes. They go to school in the bone marrow. And remember that my T cells have the receptor and my B cells have the antibody. And they're born in the bone marrow. While they're there, they're like, dude, I got the... They get to pick their receptor. They get to pick their antibody. Like, what is going to be... What antigen are they going to be sensitive to? This is the shape of the antigen that they're going to make contact with. And here's the deal. They've got two classes, two. One class, it's called positive selection. And when they go to positive selection school or class, when they go to their positive selection class, this class is to test whether or not they can bind to MHC2. So the question is, can you, are you capable of binding to MHC2 who had MHC2? The antigen presenters. If you can't bind to someone's MHC2 platform, if you can't get close enough to make contact, you're never going to be able to be activated. Remember how I said, they all have to bind on an MHC2 platform in order to be activated. Positive selection means you can do it. Another word for this is if you pass, if you pass, you are immunocompetent. And that just means you are capable of making contact and becoming activated. It's possible to be activated. If you can't bind to MHC2, you will never be activated. Let's find this out now. Do you want me to tell you how many of our students pass? Five percent, five percent of all the T and B lymphocytes, those little teenagers that show up at school, five percent of them pass. Do you know what happens to the ones who fail? This is horrible. They have to commit apoptosis. What was that again? Cell suicide. So the stakes are really high here. You can imagine if 95% of my students failed, and I, oh, I don't even like thinking about that. Let's just, yeah, I'd be fired for lots of reasons. Okay, that's enough on that thought. How about thinking about the other class that they have to take? The other class was negative selection. Now, think about what requires them to check. Do they bind to self? Do you bind to your own self-molecules? Do you think you're going to pass the class if you do? Dude, no. You're going to fail if you bind to yourself. That, if you bind and activate and attack and kill your own self, if your little receptor, your antibody or your T cell receptor, binds to self-molecules, that's actually what happens when an autoimmune disease, you recognize yourself as a bad guy. That's terrible. Like, let's not do that. If you pass the negative selection test, you are self-tolerant. Both of these things are good. Got more bad news. Be grateful that you are not a baby lymphocyte heading off to school. You're off to the Hunger Games. Like, you send your kids off to the Hunger Games, and like, you know that your kid is going to die. Ugh, whose idea is to think about these analogies? This is terrible. 5% of them pass the test. The other 95%, what happens to them? A poptosis. Sorry for your luck, dog-pound clowns. They didn't make it. Your little receptors, it was a bad choice. You picked a bad antibody, it binds to yourself, and 95% of them pick a bad one. Sad story. All right, should we take it for 10? Let's just go with the 5%. We only care about the 5%, anyway, right? So we're going to go with the 5% of B cells, and we are the 5%, and the 5% are part of humoral immunity. So let's find out what happens to these guys. Once they pass, they survive school, they probably have a little party afterwards with the rest of the 5%, because, you know, they're definitely in an elite class. And now, how does humoral immunity work with our B cells?