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:
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.
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COMPLEMENT RECEPTORS
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RECEPTOR
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LIGANDS
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FUNCTIONS
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CR1 (CD35)
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C3b, C3bi, C4b, C4bi
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Opsonization and antigen clearance (phagocytes), Antigen persistence (FDC), Immune complex clearance (RBC),Complement regulation
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CR2 (CD21)
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C3d, C3dg, C3bi, EBV
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B cell activation (B cells, FDC)
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CR3,(MAC-1, CDllb/CD18)
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C3bi
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Adhesion, extravasation, phagocytosis (macrophages, neutrophils)
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CR4 (pl50.95, CDllc/CD18)
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C3b
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Adhesion, extravasation, phagocytosis (macrophages, neutrophils)
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ClqR
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Clq
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Immune complex binding to phagocytes, (macrophages, neutrophils)
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C5aR
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C5a
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Adherence, phagocytosis, CR1 and CR3 expression (macrophages, neutrophils)
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C3aR, C4aR, C5aR
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C3a, C4a, C5a
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Mast cell degranulation Smooth muscle contraction
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