· Intro
· Activation process
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Definition |
· Series of 11 zymogens i.e. inactive precursors · Part of innate humoral immune system · Produced by the liver |
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Key features |
· Inhibition by regulator proteins e.g. C1 esterase inhibitor (deficient in hereditary angioedema) · Multiple activation pathways converge at cleavage of C3 into C3a and C3b · Activation results in formation of membrane attack complex (MAC) |
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Classical pathway |
· Activation by o Antigen-antibody complexes (IgM >> IgG) o Aggregated antibodies o CRP · C3 convertase: C4b2b |
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Alternative pathway |
· Continuous low level activation · Spontaneous thioester hydrolysis of C3 · C3 convertase: C3bBb |
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Lectin pathway |
· Homologous to the classical pathway · Mannose-binding lectin is the opsonin · Mannose present on surfaces of parasites, Candida, viruses etc. · C3 convertase: C4b2b |
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Final common pathway |
· End result of each activation pathway · C5b6789 combine to form MAC: o Membrane pore -> lysis o Opsonisation for phagocytosis by neutrophils, macrophages o Chemotaxis o Mast cell degranulation |