Complement activation (Homo sapiens)

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The complement system is a biochemical cascade that helps, or complements, the ability of antibodies to clear pathogens from an organism. It is part of the immune system called the innate immune system that is not adaptable and does not change over the course of an individual's lifetime. However, it can be recruited and brought into action by the adaptive immune system. The Classical pathway of activation of the complement system is a group of blood proteins that mediate the specific antibody response. [source: Wikipedia] It begins with circulating C1 binding to an antigen on the surface of a pathogen, which goes on to active and recruit 2 copies of each C1R and C1S, forming a C1 complex. The activated C1 complex cleaves C2 and C4. Activated cleavage products C2A and C4B combine to form C3 convertase, which cleaves C3. The cleavage product C3B joins the complex to form C5 convertase, which cleaves C5. The cleavage product C5B joins C6, C7, C8 and multiple copies of C9 to form the Membrane Attack Complex, which forms a channel for water to flood into the target cell, leading to osmotic lysis.