the complement system it involves a series of proteins that are found in the blood that are part of the immune responses but what exactly are they and how do they function by the end of this video you're going to have a better understanding of all of that so let's do it [Music] [Applause] hey this is leslie samuel here from interactive biology tv where we're making biology fun we create videos to help you understand and enjoy biology and right now we're jumping into the complement system so we have a set of proteins these proteins were made in the liver and then they get sent to the blood they're floating around waiting for something to trigger them to become active now the names of these proteins are going to be pretty easy to remember they're complement proteins so we'll use the c and then we'll give them a number we have c1 c2 all the way to c9 see biologists can make some things easy to remember this is one of those things yay okay so with the complement system we have a classical pathway and an alternative pathway let's deal with the classical pathway first because once we know that one we actually know both yeah we're just going for all easy today i want you to think about this like a cascade of events one event triggers the next which triggers the next etc etc once it starts it goes all the way to the end it's like you line up a bunch of dominoes you push one down and once you pop it won't stop all right let's start at the beginning a pathogen gets into your system the pathogen has antigens on its surface that's recognized by an antibody the antibody binds to the antigen now when that happens c1 the first complement protein will bind to the antigen antibody complex and get activated so it's activated popped and no stopping now when that c1 is activated it's gonna cause c2 and c4 to split in two okay so now we have two fragments for c2 and two fragments for c4 one of the fragments of c2 will combine with one of the fragments of c4 to form an enzyme called c3 convertes now when that happens c3 converters will split in two and form c3a which is smaller and c3b which is a larger fragment so now we have these two fragments well the smaller fragment will then diffuse out and then attract other phagocytes into the area these are the cells that come in and will start eating up pathogens to try to get rid of them so the smaller c3a fragment is like a chemical attractant that serves as a signal to bring these other recruits in so that they can help out okay so we also have the larger c3b fragment this c3b fragment will bind to the surface of the pathogen this basically tags the pathogen for destruction but that's not the only way it helps to destroy the pathogen c3b will also cause c5 to split in two to become c5a and c5b and then for the final step in the process c5b will join with c6 c7 c8 and c9 to form what's called the membrane attack complex this complex will then attach to the membrane of the pathogen and make a hole in it and when that happens water is going to rush into the pathogen causing it to break apart it's like what happens when you make a water balloon but instead of you stopping you keep filling it and filling it and filling it with water and eventually there's so much pressure that it pops well that's what happens to that pathogen thanks to the complement system so as we see we have this series of proteins forming this cascade of events that all help to just try and get rid of whatever that pathogen is now this pathway is called the classical pathway and it starts all the way at c1 binding to an antigen antibody complex but there's also an alternative pathway the cool thing is you already know it it's basically what we just went through but it skips all the antigen stuff it skips c1 c2 c4 and it starts with c3 c3 gets activated by binding directly to the pathogen itself once it binds to the pathogen it's going to interact with other proteins factor p b and d that interaction causes c3 to split into c3a and c3b and from there the rest of the pathway continues just as we discussed earlier so the classical pathway starts with the antigen antibody complex which means it's part of the specific immune response the antibody recognizes a specific antigen and triggers this complement cascade starting with c1 all the way down to c9 and the alternative pathway isn't specific c3 binds directly to the pathogen and triggers the rest of the cascade in both pathways we are recruiting other phagocytes we're tagging the pathogen for destruction and we're forming the membrane attack complex to destroy the pathogen by making that hole those holes in the membrane and filling it up with water so much that it pops man this is beautiful stuff i mean not for the pathogen but it's amazing to see how our bodies are so optimized to protect us from bad stuff and i just love it okay that's it for this video in the next one we're gonna dig into t-cells and their role in adaptive immunity peace