 the humoral response, ready? Who did it? The B lymphocytes. They're the ones who are going to make this happen. So let's draw a picture of a B lymphocyte. Remember, who is this guy? Who is this guy? I think he is. He's got antibodies. He picked his antibodies. He went to school. He passed. Holy cow. Way to go, man. You're one of the 5%. This particular antibody bound to the MHC2 platform and did not bind to self. So this little B is just hanging out. At this stage in the game, it's considered a naive B cell, NAIVE, with two dots above it. That's a naive B cell. Naive. It has had no experience at all. And it's going around at this stage in the game looking for its antigen. Now, what if magic happens and it's antigen, it finds its antigen. There's the specific antigen that it can bind to. It could be attached to a bacteria. It could be just floating in the muck. I mean, who knows, but it finds its antigen. How does it increase the chance of finding its antigen? There are a couple strategies. First of all, it hangs out in lymph nodes. All the fluid in your body goes through your lymph nodes. And so the lymphocytes go there. They go to hang out. They go to see if anybody else is there. Do I need to be activated? Have you seen my antigen? Also, the digestive system. In fact, 80% of your lymphocytes hang out in your digestive tract. Why? Because that tube that starts here and ends there, it's outside your body, dogs, everybody. And so, yeah, let's put the warriors, the patrols, the policemen, lining that tube that's going through the middle of you that's really external environment. To make sure that, you know what, if we're absorbing stuff like mad from the food that we eat, let's make sure that if we absorb any funky antigens, we have somebody there who's going to pick up, who's going to bind to that antigen and be able to initiate an immune response. Okay. So my naive B cell finds its antigen. That's the first thing that's going to happen. And it's going to bind to its antigen. And then it's interesting because you would expect, oh, now it can go do something phenomenal, not yet. It endocytosis, endocytosis, that's a verb, its own antibody with the antigen. And then guess what it does? Do you remember what kind of platforms the B cells had? It posts the antigen on MHC2. This is what we just talked about, of how the MHC gets barfed out into the cell membrane, but now the MHC2 on this naive B cell is stuck to its own specific antigen. Now you'd think, dude, do something, like come on, let's make something happen here. Not yet. You have to be activated. And guess who comes to act? Helper T cell. A helper T cell is going to come into the mix. Dude, the immune system is so awesome. This helper T cell has to have also bound to the antigen, same antigen. Remember T cells have their own special receptors. Gotta have the right color, you know. Here's my little special receptor and look what I'm going to do to my receptor to show you that the receptor binds to the same antigen. So look, the antigen, that's step four. Helper T cell binds to the antigen presented on MHC2. Helper T cell, let's go black, and that would be number four. Helper T cell binds to antigen on MHC2. If the antigen is not presented on MHC2, helper T cells, like, dude, I don't care if you bound with your antibody to the antigen, so what? You have to present that on your MHC2 platform before I will even think about activating you. But guess what happens then? Activation. How's that happen? Who cares? It's magic chemical reaction when this binding happens. Now my naive B cell is Superman. Now it is activated. Yup, that was number five. Activation. Okay, so what? What happens when you get activated? Dude, it's time for some nuclear warfare because the naive B cell starts to divide through a process called clonal expansion. And that tells you what is happening. It's like, do you remember Army of the Clone Wars? Clone Wars. Star Wars, whatever number that was, where you look out and there was like storm troopers and more like bajillions of them, that's what happens. It's storm trooper mad production. Mitosis like mad. Exponential growth of my guy here who is no longer naive. You start producing this army of B cells and their specific kind of B cells because guess what they're going to do? They're called plasma cells. This is just my naive B cell got activated and went through clonal expansion to produce plasma cells. And these guys, I mean sometimes I'm just speechless. This is so unbelievable. These plasma cells, their whole job is what? To make antibodies. And do you know how many they make? Oh, only 2,000 antibodies per second. What? They make 2,000 antibodies per second. It's out into the blood. And do you remember, like what do antibodies do? Antibodies are going to go around and bind to anything that has the antigen and they're basically like a flag that says, hey, anybody, anybody, anywhere, come on over here and eat this thing. This thing's crazy, eat it. And the macrophages and all the other phagocytes are going to come along and they're going to be like, yum-chalize. It's like a giant, like delicious morgis borg. All the macrophages are like, it's Templeton the rat at the fair. That's how excited they get when clonal expansion occurs and plasma cells start barfing out antibodies. That's just possible that it can get cooler. Dude, it just got cooler because they also, there's many of them, but some of them turn into memory cells. And if I had done any kind of preparation for this, I would sing memories for you right now. But I can't remember the tune or else I totally would. It's that late at night. The memory B cells, they're hanging out. They're not plasma cells, but you know what? Activation, huh? Who cares? Your system for the rest of your life. And if you get exposed again, memory B cells are going to differentiate, divide into plasma cells like instantly. We know this threat is nuclear bomb worthy. So why wait? Why go through that long process again? The memory cells are ready to go and we're going to activate and like plasma cells, antibody, madness, storm, troopers, everywhere, a lot faster the second time. Hopefully your brain is thinking, this could come and handy this whole memory phenomenon. So in the next section, we'll talk about why that actually comes in super crazy handy.