II. Physiology
- Complement are components of humoral immune system
- Bind to aggregated IgG
- Undergo proteolysis
- Trigger inflammatory response (Chemoattractant)
- Solubilizes immune complexes to aid in clearance
III. Labs
- Complement C3
- Marker for Intrinsic and Extrinsic Pathway Function
- Measured by immunochemical assay
- Complement C4
- Marker for Intrinsic Pathway Function
- Measured by immunochemical assay
- Deficient in 1% of population
- Deficient in 11% with Systemic Lupus Erythematosus
- Complement CH50
- Marker for function of entire intrinsic cascade
- Measured by serum ability to lyse IgG coated RBCs
- Most affected by delay in performing assay
IV. Low Complement levels indicate depletion
- Rheumatic causes
- Systemic Lupus Erythematosus
- Mixed connective tissue disease (MCTD)
- Vasculitis (especially cryoglobulinemia)
- Non-Rheumatic Causes
- Septic Shock
- Liver failure
- Severe malnutrition
- Pancreatitis
- Severe burns
- Atheromatous embolization
V. Indications: Follow rheumatic disease activity
- Complement fall 20% below baseline signal exacerbation
