Complement Pathways: Classical, Lectin & Alternative

Initiated by mannan-binding lectin on microbes, activating C4 and C2.

A complex similar to C1qrs, initiates this cascade upon lectin binding.

Cleaved into C4a and C4b during the Lectin and Classical pathways.

Formed by C4b and C2a, cleaves C3 into C3a and C3b

Formed by C4b2a and C3b, cleaves C5 into C5a and C5b.

Initiated by antigen-antibody complexes.

First component of Classical Pathway, binds to antibodies complexed with antigens.

A convertase that cleaves C3 into C3a and C3b in the classical pathway.

A convertase that cleaves C5 into C5a and C5b in the classical pathway.

Forms a complex inserting into the pathogen membrane

Activated spontaneously, involving factors B, D, and Properdin.

Factor that stabilizes the C3bBb complex in the Alternative Pathway.

Continuously formed by spontaneous hydrolysis of C3.

Fluid-phase C3 convertase in the Alternative Pathway.

Membrane-bound C3 convertase in the Alternative Pathway.

C3 convertase of the alternative pathway bound to properdin.

Anaphylatoxin; induces inflammation, recruits immune cells.

Opsonin; coats microbes, facilitating phagocytosis.

Anaphylatoxin; potent chemoattractant and activator of neutrophils.

Ultimately forms the membrane attack complex (MAC).