Fig. 6.

Diagram showing some of the key elements of the complement cascade. In the classical pathway, initiation occurs when C1q binds to IgG or IgM immune complexes. The lectin pathway engages when mannose-binding lectin (MBL) binds pathogen associated carbohydrate moieties, such as mannose or glucose. In the alternative pathway, C3 spontaneously hydrolyzes (dotted line) but negative regulators (e.g., CFH and CFI) bound to the extracellular matrix of host cells prevent amplifying cleavage of C3 by C3b (dashed line). Without binding opportunities for CFH, as on bacterial cell walls, positive feedback occurs ultimately leading to MAC deposition. Figure adapted from Thurman and Holers (2006).