Formation of C3 Convertase and C5 Convertase during
the Classical Complement Pathway

The MBL-associated MASP2 is able to cleave C4 into C4a and C4b. The C4b binds to adjacent proteins and carbohydrates on the surface of the antigen. C2 then binds to the C4b and MASP2 cleaves C2 into C2a and C2b. The C4b2a functions as a C3 convertase that can subsequently cleave hundreds of molecules of C3 into C3a and C3b. C3b attaches antigens to phagocytes for opsonization (enhanced attachment) while C3a can promote inflammatory responses that enable body defense cells and defense chemicals to leave the blood and enter the tissues.

Some of the C3b combines with the C4b and the C2a. C4b2a3b functions as a C5 convertase that cleaves molecules of C5 into C5a and C5b. C5a is the most potent complement protein triggering inflammation. C5b becomes part of the Membrane Attack Complex (MAC).

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