Fig. 5: Formation of C5 Convertase during the Classical Complement Pathway and Cleavage of C5 by C5 convertase

C4b2a3b functions as a C5 convertase that cleaves C5 into C5a and C5b. C5a is the most potent complement protein triggering inflammation. It causes capillary vasodilation and also binds to mast cells causing them to release vasodilators such as histamine to increaset blood vessels permeability; it increases the expression of adhesion molecules on leukocytes and the vascular endothelium so that leukocytes can squeeze out of the blood vessels and enter the tissue (diapedesis); it causes neutrophils to release toxic oxygen radicals for extracellular killing; and it induces fever. C5a also functions as a chemoattractant for phagocytes. Leukocytes will move towards increasing concentrations of C5a. C5b becomes part of the Membrane Attack Complex (MAC).

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