Complement cascade

Three pathways

·      Classical: activated when antigen-antibody complexes form

·      Alternative

·      Mannose-binding lectin

The alternative and mannose-binding types are activated by C3 hydrolysis or antigens without the need for antibodies.

C3-convertase activates C3, forming C3a and C3b

·      C3a – C3a and C5a cause degranulation of mast cells, increasing vascular permeability and increasing smooth muscle contraction.

·      C3b – binds to surface of pathogens (opsonization) enhancing phagocytosis

The end product of the complement cascade is membrane attack complex which form transmembrane channels and cause osmotic lysis of the cell.