In this video, we're going to briefly talk about the proteins of the complement system. The complement system proteins are all designated with a letter C and are numbered C1 through C9 in terms of the order of their discovery, where C1 was first and C9 was discovered last. These complement system proteins remain active and circulate through our blood and in our tissues as their inactive forms until these complement system proteins are fragmented. Once these complement system proteins are fragmented, they become activated. The activated fragments are indicated by lowercase letters, such as for example, the lowercase letter 'a' and the lowercase letter 'b'.

The complement system protein C3 is a very important and critical protein. These complement system proteins exist in their inactive forms, and so C3 is a critical inactive complement system protein. The C3 complement system protein can become activated by the activity of an enzyme called C3 convertase. C3 convertase is an enzyme that activates C3 by hydrolyzing or fragmenting C3 into the C3a and C3b fragments. The C3a and C3b fragments, which are active, can interact with other complement system components, ultimately causing some kind of immune response. Recall that the immune responses of complement system activation include opsonization, cell lysis, and inflammation, all of which we'll talk more about as we move forward in our course.

If we take a look at our image down below, notice what we're showing you here at the top is the enzyme C3 convertase, which is being represented here as a pair of scissors because its function is to enzymatically cleave or fragment C3. Notice here we have the complement system protein C3 right in the middle. C3 convertase is an enzyme that will cleave or fragment C3 into the activated fragments C3a and C3b. C3a and C3b, through a cascade of reactions, can ultimately lead to immune responses, such as for example, C3a can lead to inflammation—an innate immune response that can help to eliminate microbes—and we'll get to talk a lot more details about inflammation later in our course. C3b can lead to a process known as opsonization, which we'll talk more about moving forward in our course as well, but it helps to improve phagocytosis and can also lead to cell lysis. Here we are showing you a microbe that is being lysed.

Ultimately, what we are saying here is that these proteins of the complement system need to become activated, and upon activation, they can lead to immune responses such as inflammation, opsonization, or cell lysis. This here concludes our brief lesson on the proteins of the complement system, and we'll be able to learn more about the complement system as we continue to move forward. So, I'll see you all in our next video.