Activation of C3 and C5

The 3 pathways of complement activation converge on the cleavage of C3 by C3 convertases. C3 convertase cleaves C3 into C3a and C3b - a central step of complement activation. C3a remains in the fluid phase and acts as an anaphylatoxin, whereas C3b can form additional C3 convertases hastening the production of C3b. Besides, C3b binds to C3 convertases to form C5 convertase, which can act as an opsonin, or is degraded into fragments which cannot form an active convertase.

补体激活的三大途径最终汇聚于C3蛋白被C3转化酶裂解的过程。C3转化酶将C3裂解为C3a和C3b，这是补体激活过程中的一个核心步骤。C3a保持于流体相中，充当过敏性毒素的作用，而C3b能够进一步形成更多的C3转化酶，从而加速C3b的产生。此外，C3b还可以与C3转化酶结合，形成C5转化酶，C5转化酶既可以作为调理素发挥作用，也可能被降解为无法形成活性转化酶的片段。