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Complement

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Topic updated on 10/17/16 12:08pm

Overview
  • Complement is a system of proteins that interact to play a role in humoral immunity and inflammation 
  • Role of specific complement protein factors
    • membrane attack complex
      • C5b-9
        • defends against gram-negative bacteria
        • gram positive bacteria are resistant
          • though opsonization still effective
        • puts holes in membrane
    • opsonins
      • C3b and IgG are the two primary opsonins in bacterial defense
    • clearance of immune complexes
      • C3b
    • physiological control stages
      • C1, C3, C5
      • decay-accelerating factor (DAF) and C1 esterase inhibitor
        • help prevent complement activation on self-cells
    • anaphylaxis
      • C3a, C5a
        • recruitment of inflammatory cells
  • Complement proteins synthesized in the liver
Classic pathway
  • Antigen-antibody complex initiates
  • Both IgM and IgG can activate
    • Pentameric IgM is more efficient due to multiple binding sites
    • >2 IgG's needed for activation
  • Deficiencies
    • C1q, C1r, C1s, C4, C2
    • ↑ immune complex diseases
    • ↑ infections due to pyogenic bacteria
Alternative pathway
  • Antibody independent
  • C3b binds directly to conserved regions on bacterial surface
    • LPS of Gram neg microbes (endotoxin)
    • polysaccharides
Deficiencies
  • Involving both classical and alternative
    • C3
      • ↑ immune complex diseases
      • ↑ infections of pyogenic bacteria
    • C5, C6, C7, or C8
      • ↑ infections of Neisseria spp. bacteria
  • Involving regulatory proteins
    • C1-INH
      • hereditary angioedema
    • DAF
      • paroxysmal nocturnal hemoglobinuria (PNH)

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This video gives an overview of the complement system.
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