II. Physiology

  1. Complement are components of humoral immune system
  2. Bind to aggregated IgG
  3. Undergo proteolysis
  4. Trigger inflammatory response (Chemoattractant)
  5. Solubilizes immune complexes to aid in clearance

III. Labs

  1. Complement C3
    1. Marker for Intrinsic and Extrinsic Pathway Function
    2. Measured by immunochemical assay
  2. Complement C4
    1. Marker for Intrinsic Pathway Function
    2. Measured by immunochemical assay
    3. Deficient in 1% of population
    4. Deficient in 11% with Systemic Lupus Erythematosus
  3. Complement CH50
    1. Marker for function of entire intrinsic cascade
    2. Measured by serum ability to lyse IgG coated RBCs
      1. Most affected by delay in performing assay

IV. Low Complement levels indicate depletion

  1. Rheumatic causes
    1. Systemic Lupus Erythematosus
    2. Mixed Connective Tissue Disease (MCTD)
    3. Vasculitis (especially cryoglobulinemia)
  2. Non-Rheumatic Causes
    1. Septic Shock
    2. Liver failure
    3. Severe Malnutrition
    4. Pancreatitis
    5. Severe burns
    6. Atheromatous embolization

V. Indications: Follow rheumatic disease activity

  1. Complement fall 20% below baseline signal exacerbation

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