Veterinary Immunology and Serology

REGULATION OF COMPLEMENT SYSTEM

• The regulation is accomplished through several regulatory proteins.
• C₁– INH (C₁ inactivator) - It is serine protease inhibitor and present in plasma. It binds to C₁r and C₁s and dissociate them from C₁q thus control assembly of C_4b2b(Classic pathway)
• Factor - I: It is a serine protease and present in plasma. It cleaves C_3b and C_4b by using factor H and MCP (membrane cofactor for protein), C₄BP or CR₁ (Type 1 complement receptor) as co factors.
• Factor – H: It as a plasma protein binds C3b and displaces BP. It is a cofactor for factor I- mediated cleave of C_3b
• C₄ binding protein (C₄BP): Binds C_4b and displaces C₂. It acts as cofactor for factor I mediated cleaves of C_4b.
• Membrane cofactor for protein (MCP): It is present in leukocytes, epithelial cells and endothelial cells. It acts as cofactor for factor I mediated cleavage of C_3b and C_4b
• Decay accelerating factor (DAF): present in blood cells, endothelial cells and epithelial cells. It displaces C2b from C4b and Bb from C3b.
• Vitronectin, clusterin or Protectin: Present in Blood cells, endothelial cells and epithelial cells. They block C₉ binding and prevent formation of MAC (membrane attack complex).