 So, compliment is not just a nice thing that you say to somebody. Compliment is a, like a cascade of crazy chemical reactions. Like, I don't know, there's like 25 intermediates in, that results in something called the membrane attack complex or MAC attack. Okay, just MAC, I added the attack. Because doesn't MAC attack sound cool? So compliment, 25 proteins, crazy madness, what stimulates compliment? Well, some invading critter, like a crazy bacterium, has some little antigen marker that initiates compliment. So it will stimulate, it will connect to one of the bunch of different chemicals that are involved in this cascade. Now, of course, as is always the case, cascades are wholly more complicated than the way that we draw them. Doesn't really look like a little boing-boing arrow. What is the membrane attack complex? Everything's awesome. Compliment, all these different proteins get formed and they branch and some of them, some of the proteins in this cascade actually are inflammatory cytokines, so they're going to initiate more inflammation, which is awesome, bring more guys to help do battle against this bad guy right here. And ultimately, a bunch of the proteins involved in the cascade put themselves together to form like this crazy, it's almost like a channel that gets formed inside the cell, the bad guy cell and it pops the bad guy. The MAC attack complex pops the bad guy. The MAC membrane attack complex basically pokes a hole in the invading cell and then the hole allows stuff to go in and out and the cell can no longer survive. There's something else really cool that I wanted to tell you, but before I'm going to show you a picture, I have a picture to show you as well. Compliment is part of the innate immune system. However, antibodies, I've told you that antibodies are part of the acquired immune response and also part of the innate immune response. And this is an example of how antibody function can carry over. So look, I made this like kind of wiggly looking thing, so I'm going to make kind of a little square. If this antibody matches, and antibodies really are specific, but if the antibody matches with an antigen on the bacterial invader or whoever the invader is, the antibody, when it binds to its antigen, that it matches. Now remember, this little end piece is super specific. It's not going to match with just anybody, but if the antibody matches, then there's a little place, it's crazy, in the armpit of the antibody. Seriously, that normally the antibody shape covers that place. It's not available. But if the antibody binds with the antigen, then this complement binding site, that says complement, binding site opens up. And complement proteins can bind to the antibodies and can enter the cascade or stimulate the cascade to begin. There's essentially an antibody that finds its antigen, can initiate complement. That's a great strategy because let's have this, let's pop the bad guy that we just found our match for. It makes perfect sense. Now look at this image. I think this helps. Okay, so here's your bad guy. Here's the antigen. The antigen binds to the antibodies. Antibodies actually have, it doesn't show it in the armpit. It's way cooler when it shows it in the armpit. But you can see that the antibody has initiated complement. Now, each one of these is a different complement protein, and we have multiple actions. We could totally memorize the whole thing. Yeah, that's not going to happen. But look at all these actions that happen. Everybody, like they all have cool roles, they all work together, they break apart, they cause different things, but look at the end game. What? They come together and they form, like look at what this says about it. A fragment from C5, which was one of the complement proteins, joined C6, C7, C8, and C9. So all these different complement proteins come together to form the Mac attack. And then you get a hole in the bad guy and we pop. The complement is amazing. Compliment isn't specific, but look at that overlap. Antibodies can initiate complement. Next up is a non-specific immune system.