| Themes > Science > Life Sciences > General Biology > Immunology > The Complement System > The Biological Effects of Complement |
1. Opsonisation. The C3b and, to a lesser degree, C4b molecules are opsonins. That is they coat foreign organisms either by the AP or those already bound by antibody. Opsonisation of particles greatly enhances their phagocytosis by means of binding to specific complement receptors (see below). 2. Inflammation. The C5a and, less potently, the C4a and C3a fragments are important inflammatory activators inducing vascular permeability, recruitment and activation of phagocytes. 3. Lysis. C5b binds and recruits C6 and C7 to the target surface. C7 and subsequently C8 change conformation to expose hydrophobic domains which insert in the lipid bilayer. The C5b678 complex catalyses the polymerisation of the final component C9 which forms a transmembrane pore of ~ 10nm diameter causing lysis of the cell. This macromolecular assembly is known as the Membrane Attack Complex (MAC). 4. Immune complex clearance. Complement has a very important role in solubilising and causing removal from the circulation of immune complexes. It does this by the binding of C4b and C3b, covalently bound to the immune complex, to CR1 complement receptors on red blood cells which transport the complexes to the liver and spleen where they give the complexes up to phagocytes for destruction. |
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