In this video, we're going to do a review of the complement system. And so, because this video is a review, what that means is that there's really no new information here in this video. And so, if you're already feeling really good about the complement system then you can feel free to skip this video. But if you're looking for a good way to review a lot of the concepts that we talked about in our previous lesson videos, then stick around because this might be a really helpful review for you. In reviewing the complement system, we're going to review the activation pathways, as well as the responses or the effects of activation of the complement system. Recall that the complement system consists of inactive proteins that are circulating in our blood and circulating through the tissues. These inactive complement system proteins can become activated by one of three different pathways: the alternative pathway, the lectin pathway, or the classical pathway. Now, the alternative pathway will activate the complement system and is triggered by C3b, a complement system fragment, binding to the surface of invading microbes. When C3b binds to the surface of invading microbes, it helps to activate the complement system. Of course, the alternative pathway is going to ultimately lead to the formation of the enzyme C3 convertase, which is actually where all three of these pathways converge together. So, the lectin pathway is going to be triggered and activate the complement system through the MBL, the mannose-binding lectin. The mannose-binding lectin consists of a protein that binds to mannose carbohydrates that are found on the surface of microbes. So they bind to the surface of invading microbes by binding to the mannose on those microbes. Here in this image, notice that the mannose is represented by these little green hexagons that are on the surface of the microbe. The lectin, the mannose-binding lectin or the MBL, is represented by this structure that you see here. When the mannose-binding lectin binds to the mannose on the surface of the microbes, it will lead to a cascade of events that ultimately leads to the formation of C3 convertase. Then, the classical pathway is going to be triggered and activate the complement system when antibodies from the adaptive immune system are going to bind to antigens on the surface of microbes. Ultimately, these antibodies from the adaptive immune system that bind to the surface of these microbes lead to a cascade of events that once again leads to the formation of C3 convertase. This classical pathway here has an asterisk next to it because the classical pathway is really why the complement system is called the complement system. Because through the classical pathway, the complement system, which is part of innate immunity, is able to complement the adaptive immunity. That's because the antibodies are part of adaptive immunity, and the antibodies help to trigger the complement system, which is part of innate immunity. There's some crosstalk between innate immunity and adaptive immunity right here through the classical pathway. But regardless of which pathway is used, the alternative lectin or classical pathway, they all lead to the formation of C3 convertase, which is an enzyme that is being represented as scissors here because this enzyme C3 convertase will cleave or fragment this inactive complement system protein called C3. When C3 is cleaved by C3 convertase, it will be cleaved into C3a, which is over here, as well as C3b, which is over here. And so there will be high levels of C3a and high levels of C3b after the formation of C3 convertase. Now, the C3a, along with other complement system proteins such as, for example, C5a, can go on to trigger the inflammatory response to help eliminate microbes. And C3b can go on to act as an opsonin to allow for opsonization, which is going to improve the effectiveness of phagocytosis, allowing phagocytes like macrophages to phagocytose microbes much easier and much more effectively. And C3b can also interact with a series of other complement system proteins to lead to microbe cell lysis through the formation of membrane attack complexes that form pores in the membranes of gram-negative bacteria, such as these bacteria being shown here. And so this here concludes our review of the complement system, and we'll be able to apply some of the concepts that are in this review as we move forward. So I'll see you all in our next video.