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The classical and alternative complement pathways.

There are two forms of C3-convertase (EC

  • The second consists of the C3bBb complex, which forms during the alternative complement pathway. It is formed when membrane bound C3b binds to factor B which is in turn cleaved by factor D in Ba and Bb. Bb remains bound to C3b while Ba is released into the surrounding medium.

Once formed, both C3-convertases will catalyze the proteolytic cleavage of C3 into C3a and C3b (hence the name "C3-convertase"). C3b can then act as an opsonizer or itself become a C3-convertase.

A convertase (of either type) with an additional 3b (C4b2b3b or C3bBb3b) is known as "C5-convertase".

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