Fig. 1

Classical and lectin pathways of complement activation [5]. The classical pathway of complement activation (top left) is initiated by the binding of C1q to antigen-antibody complexes. In contrast, the lectin pathway (top right) is initiated when collectins (including mannose-binding lectin, or MBL) or ficolins, in association with MBL-associated serine proteases (MASPs), bind to targets on cell surfaces. Both pathways, along with the alternative pathway (not shown), ultimately result in formation of C3b, which opsonizes pathogens, and anaphylatoxins C5a and C3a, and results in clearance via phagocytosis (bottom center) or through lysis of membranes via the terminal pathway