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A complex system consisting of approximately 30 different proteins
Complement System
Result of activated complement
Inflammation, increased phagocytosis, and kills some bacteria and viruses
Complement
A cascade reaction with sequential interaction of the components
Steps of complements
Activation, amplification, and biological function
Complement major function
Opsonization of microbes, release of inflammatory and chemotactic mediators, and direct lysis of microbes
Three main pathways of complement activation
Alternative, Lectin, and Classical
End effects of complement activation
Increased phagocytosis, inflammation, and cell lysis
Alternative pathway of complement activation
1. C3 combines with factors B, D, and P on the surface of a microbe
2. This causes C3 to split into fragments C3a and C3b
3. Three different outcomes can occur resulting in the death of the microbe
Role of C3a fragment
C3a recruits and activates leukocytes, leading to inflammation and destruction of leukocytes
Role of C3b
Bound C3b is recognized by phagocyte C3b receptor and phagocytosis of the microbe occurs
Role of C3b formation of membrane attack complex (MAC)
Formation of membrane attack complex (MAC) occurs, leading to lysis of the cell
Classical pathway of complement activation
One of three pathways of complement activation. It is activated by antibody bound to antigen, and involves complement components C1, C4, and C2 in the generation of the C3 and C5 convertases. See also alternative pathway of complement activation; lectin pathway of complement activation.
Lectin pathway of complement activation
One of the three pathways of complement activation. It is activated by the binding of a mannose-binding lectin present in blood plasma to mannose-containing peptidoglycans on bacterial surfaces.
Activation triggers for the classical pathway of the complement system
1. Antigen + IgG or IgM
2. Microbe + CRP
Activation triggers for the alternative pathway of the complement system
Microbial/cell surface
Activation triggers for the lectin pathway of the complement system
Microbe + MBL
Outcome of MAC
Target lysis
Action of MAC
Membrane destabilization and lysis of target cells
Outcome of C3a and C5a
Tissue inflammation
Action of C3a and C5a
Vasodilation- by activation of mast-cell degranulation - release of vasoactive amines
Chemotaxis- leading to migration of neutrophils, eosinophils, and macrophages to the site of C activation
Outcomes (2) of C3b
Opsonization and Immune complex clearance or adherence
Action of C3b (Opsonization)
Enhanced phagocytosis by neutrophils and macrophages
Action of C3b (Immune complex clearance or adherence)
Binding of immune complexes through CR1 on erythrocytes and transport to spleen and liver for clearance