Complete action of complement enzymes in immune reactions can be studied in three phases.

1. Recognition and activation phase:

The first stage is recognition and activation phase. In this phase components of complement system recognize cell bound antibodies or antibody in immune complex and get activated.

2. Enzymatic phase:

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Enzymatic phase is characterized by amplification of complement enzymes.

3. Attack phase:

Attack phase shows the formation of membrane bound complement complex (MAC) for attack on the target cell.

COMPLEMENT RECEPTORS

RECEPTOR

LIGANDS

FUNCTIONS

CR1 (CD35)

C3b, C3bi, C4b, C4bi

Opsonization and antigen clearance (phagocytes), Antigen persistence (FDC), Immune complex clearance (RBC),Complement regulation

CR2 (CD21)

C3d, C3dg, C3bi, EBV

B cell activation (B cells, FDC)

CR3,(MAC-1, CDllb/CD18)

C3bi

Adhesion, extravasation, phagocytosis (macrophages, neutrophils)

CR4 (pl50.95, CDllc/CD18)

C3b

Adhesion, extravasation, phagocytosis (macrophages, neutrophils)

ClqR

Clq

Immune complex binding to phagocytes, (macrophages, neutrophils)

C5aR

C5a

Adherence, phagocytosis, CR1 and CR3 expression (macrophages, neutrophils)

C3aR, C4aR, C5aR

C3a, C4a, C5a

Mast cell degranulation Smooth muscle contraction