The three pathways of complement activation.

<p>The alternative pathway (AP) is activated when C3 undergoes spontaneous hydrolysis and forms the initial C3 convertase, C3(H₂O)Bb in the presence of factor B (fB) and cleavage of bound fB by factor D (fD). The alternative C3-convertase is stabilized by properdin (P). The C3 convertase generates C3b and the subsequent C3-convertases are assembled by C3b and Bb. The lectin pathway (LP) is activated when MBL or other immune lectins bind carbohydrates on pathogens, activating the associated serine proteases (MASPs) which cleave C4 and C2. The first component of the classical pathway (CP) is C1, a complex of C1q and its associated serine proteases C1r and C1s. The CP is initiated by C1q recognition of immune complexes, activating C1r and C1s, again cleaving C4 and C2 generating a C3 convertase characteristic of the CP and the LP (C4bC2a). The C3 convertase cleaves C3 generating C3b and enables assembly of a C5 convertase (C3bC3bBb or C4bC2aC3b), and the C5-convertase product (C5b) initiates assembly of the membrane attack complex (MAC) from C5b-C9. * Initial alternative C3 convertase generated from C3(H₂O) and fB.