Complement

C1q and factor B play a central role in the activation of the alternative and classical pathways respectively.

C3 occupies a pivotal role in all three complement pathways:

• Classical
• Alternative
• Lectin

The membrane attack complex follows from C3 activation and so is common to all of these pathways.

Although C3 is central to activation of both classical and alternative pathways, it is not a component of the membrane attack complex, which comprises C5-9.

C3a and C5a are both potent anaphylotoxins, causing vasodilation, leukocyte chemotaixs and increased vascular permeability.

In man, immune complexes that have activated complement bind to complement receptor 1 on red blood cells. Smaller immune complexes that have not fixed complement may be transported in a soluble form.

Factor H is a fluid-phase inhibitor of the alternative pathway, not the classical pathway.