The Complement System

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The Complement System

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The Complement System



innate defense functions
trigger inflammation
attract phagocytes
opsonize antigens
cause cell lysis
activate naïve B-lymphocytes
remove immune complexes



the classical pathway
antibody-dependent
proteins of the classical pathway
C1
6 molecules of C1q
2 molecules of C1r
2 molecules of C1s

C3
C5

Ag-Ab complex activates C1

animation showing the assembly of C1 during the classical pathway


animation showing the assembly of C3 convertase

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animation showing the cleavage of C3 and the formation of C5 convertase


opsonization and phagocytosis [C3b]

CR1 receptors



inflammation [C5a]

the MAC and lysis [C5b6789n]

animation showing the formation of the MAC





agglutination

Type III hypersensitivity reaction

• rheumatoid arthritis
• subacute bacterial endocarditis
• immune complex glomerulonephritis
• systemic lupus erythematosus
• Arthus reaction
• Farmer’s lung
• polyarteritis nodosa

viral neutralization

chemotaxis [C5a]

animation showing the benefits of C5a and C3b

basophil/mast cell activation [C3a, C4a, C5a]

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the lectin pathway

mannan-binding lectin (MBL)–dependent

proteins of the lectin pathway

MASP1

MASP2

C4 & C2 cleaved

animation of the lectin pathway


the alternative pathway

animation of the alternative pathway and formation of C3 convertase


antibody-independent



proteins of the alternative pathway

C3

C3b

C3i

B
D
H
I
P (properdin)

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animation of the formation of C5 convertase.


large polysaccharides react with factors B and D

C3bB

C3bBb

formation of activation product that activates C3

Bb

C3bBbP

regulation of complement activity
factor H
factor I
C1 inhibitor (C1INH)



complement receptors
CR1


CR2
CR3
CR4

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disorders of the complement system


C3 deficiency
C2 deficiency

immune complex disorders

systemic lupus erythematosus (SLE)

C9 deficiency
C1INH deficiency

hereditary angioneurotic edema (HANE)

decay accelerating factor (DAF) deficiency

paroxysmal nocturnal hemoglobinura (PNH)