Complement Disorders

  • Complement protein deficiencies
    • Early complement deficiencies (C1-C4)
      • Increased risk of severe, recurrent pyogenic sinus and respiratory tract infections. Increased risk of SLE.
    • Terminal complement deficiencies (C5–C9) Increased susceptibility to recurrent Neisseria bacteremia.
  • Complement regulatory protein deficiencies
    • C1 esterase inhibitor deficiency
      • Causes hereditary angioedema (edema of skin, especially periorbital and mucosal surfaces) due to unregulated activation of kallikrein → ­ bradykinin.
      • Characterized by ¯ C1 esterase inhibitor and ¯ C4 levels.
      • May be triggered by blood transfusions
      • ACE inhibitors are contraindicated due to increased bradykinin levels
  • Paroxysmal nocturnal hemoglobinuria
    • Defect in the PIGA gene preventing the formation of anchors for complement inhibitors, such as decay-accelerating factor (DAF/CD55) and membrane inhibitor of reactive lysis (MIRL/CD59).
    • Causes complement-mediated lysis of RBCs.
    • See entry in intrinsic hemolytic anemia

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