C1R

Fala Sério, Mãe — 2017
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Complement Pathways

Components of complement pathways of the immune system. 

Classical Pathway: binds to the pathogen surface

  1. C1 binds to phosphocholine on bacteria, which activates C1r to cleave C1s.
  2. Activated C1s cleaves C4 to C4a and C4b.
  3. C4b binds to the microbial surface and also binds C2.
  4. C2 is cleaved to C2a and C2b by C1s, forming the C4bC2a complex.
  5. The C4bC2a complex cleaves C3 to C3a and C3b.
  6. C3b binds to the surface and causes opsonization.

MB-Lectin Pathway: uses mannin-binding lectin to bind to mannose-containing carbohydrates on the pathogen surface

  1. Mannin-binding lectin (MBL) binds to the pathogen surface and activates MASP-2.
  2. MASP-2 cleaves C4 to C4a and C4b.
  3. C4b binds to the microbial surface and also binds C2.
  4. C2 is cleaved to C2a and C2b by MASP-2, forming the C4bC2a complex.
  5. The C4bC2a complex cleaves C3 to C3a and C3b.
  6. C3b binds to the surface and causes opsonization.

Alternative Pathway: binds to the pathogen surface with spontaneously activated complement, amplifies C3b

  1. C3b deposited by the C3 convertase binds to factor B.
  2. Factor B is cleaved by factor D into Ba and Bb, forming the C3bBb complex.
  3. The C3bBb complex cleaves C3 into C3a and C3b.
  4. C3 spontaneously hydrolyzes to C3(H2O).
  5. C3(H2O) binds to factor B, and factor D cleaves factor B.
  6. Upon factor B cleavage, the C3(H2O)Bb complex is formed.
  7. The C3(H2O)Bb complex cleaves C3 into C3a and C3b.
  8. Factor B binds to C3b on the surface and is cleaved to Bb.