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Unformatted text preview: THE COMPLEMENT SYSTEM Activation of the complement protein C3 triggers several mechanisms that contribute to microbial destruction. C3 can be activated by either the classical or the alternative pathway. Activation splits C3 into two smaller pieces, C3a and C3b. This leads to three processes that help destroy microbes: a. Cytolysis---this involves damage to the plasma membrane of foreign cells. Cellular contents leak out and the cell dies. Steps (classical pathway): 1) Once antibodies attach to the antigen (foreign cell), the complement protein C1 in turn binds to the antibodies and becomes activated. 2) Activated C1 activates C2 and C4 by splitting them into fragments: C2a and C2b plus C4a and C4b. C2b and C4b combine to form an enzyme that activates C3, splitting it into C3a and C3b. . 3) C3b starts a series of reactions that activate C5 - C9. These form a membrane attack 3) C3b starts a series of reactions that activate C5 - C9....
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- Fall '09