MCQ Pathology Answer 5
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The binding of antibody to its antigen often triggers the complement system through the so-called classical pathway. It can occur in solution or when the antibodies have bound to antigens on a cell surface.
The proteins of the classical pathway
C1
C1 exists in blood serum as a molecular complex containing:- 6 molecules of C1q
- 2 molecules of C1r
- 2 molecules of C1s
- Binding of C1q activates C1s and C1r.
- Activated C1s (a serine protease) cleaves two serum proteins:
- C4 is cleaved into a large fragment
- C4b, which binds covalently to sugar residues on cell-surface glycoproteins, and a smaller, inactive, fragment of
- C4a which diffuses away.
- C2 is cleaved into
- C2b, which binds noncovalently to a site on C4b, leaving a smaller, inactive, fragment of
- C2a which diffuses away.
- The complex of C4b•2b is called "C3 convertase" because it catalyzes the cleavage of C3. (C4b•2b is also a serine protease.)
- C4 is cleaved into a large fragment
Tags: MCQ, Pathology, Complement, Inflammation
Category: Pathology MCQs
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