Schematic representation of the three complement activation pathways, converging to cleave C3. Classical pathway (activated by immune complexes), MBL-pathway (activated by bacterial sugars such as mannose) and the alternate pathway (activated even in the absence of antibody), result in formation of the C3 convertase, which culminates in C3 activation. After this occurs, C3b will opsonize the surface of the pathogen, with subsequent phagocytosis. Further progression of the cascade leads to the activation of C5-9 and to the formation of the membrane attack complex (MAC), lysing the microbe/cell. Byproducts of these pathways include anaphylotoxins C3a and C5a, recruiting leukocytes which contribute to the inflammatory response.
Miller et al. Clinical and Molecular Allergy 2010 8:6 doi:10.1186/1476-7961-8-6