Figure 1.

Overview of the Complement Pathways

The three activating pathways and the terminal pathway are shown. The acronyms of participating proteins are given. The lectin pathway is activated when MBL or ficolins in complex with MASPs recognize foreign patterns of carbohydrate or acetyl groups. The classical pathway is activated by C1q in complex with C1s and C1r upon binding of antibody-antigen complexes. The alternative pathway is constitutively activated and inhibited on self-surfaces, but is allowed to proceed on foreign surfaces. The three pathways converge on C3 and C5 and the common terminal pathway leading to formation of the membrane attack complex.