Table 1.

Functional Components of the Complement System

Functional Components	Molecules	Functions
Initiator complement factors	C1q complex Mannose binding lectin Ficolins	Trigger complement cascade by binding to activated soluble or membrane bound ligands
Enzymatic mediators	C3 convertase C5 convertase	Induce cleavage or conformational changes of complement components
Opsonins	C3b, C4b (largest fragment resulting from convertase cleavage)	Bind to specific receptors at bacterial cell surface Stimulate phagocytosis
Anaphylatoxins	C3a, C4a, C5a (smallest fragment resulting from convertase cleavage)	Bind to endothelial cells and increase blood flow Trigger cytokines cascade
Membrane attack complex	Assembly of C5b, C6, C7 and C9	Bind and destroy bacteria membrane with subsequent lysis of bacteria
Complement receptors	Cell surface receptors	Bind to complement proteins
Regulatory complement factors	Soluble: Factor H C4-binding protein Cell membrane: Decay-accelerating factor Complement receptor 2 Complement receptor 1 Membrane cofactor protein Proteolytic enzymes serine protease factor 1	Prevent overactivation of complement proteins