Your Immune System's First Responder (Consumption Marker)
Complement C3 is a protein in your innate immune system that helps destroy bacteria and clear immune complexes. In autoimmune diseases (especially lupus), C3 gets consumed as it attacks your own tissues. Low C3 = the complement system is being used up, usually by active autoimmune disease. It's like checking your fire extinguisher—if it's depleted, there was recently a fire.
What is Complement C3?
C3 is the central complement protein, common to classical, alternative, and lectin pathways. Made by the liver. Normal: 90-180 mg/dL. Low C3 indicates complement consumption (immune complex disease, active SLE) or rarely decreased production (liver failure) or genetic deficiency.
↑ What High Complement C3 Means
Acute phase reactant—C3 rises with inflammation and infection. Mildly elevated C3 is usually just inflammation.
Common symptoms:
Usually just indicates inflammation—typically no specific symptoms from C3 elevation
↓ What Low Complement C3 Means
Complement is being consumed. Most commonly from active lupus (SLE), immune complex-mediated diseases, or rarely inherited complement deficiency. Low C3 + low C4 + rising anti-dsDNA = active lupus flare.
Common symptoms:
Symptoms of the underlying disease · If lupus flare: fatigue, joint pain, rash, fever, kidney involvement · If infection: recurrent sinopulmonary infections (if genetic deficiency)
Why It Matters
When normal:
Monitors lupus disease activity (low = active)
Low C3 + C4 with rising anti-dsDNA = impending or active flare
Identifies immune complex-mediated disease
Inherited C3 deficiency is a rare cause of recurrent infections
Risks if abnormal:
Low: active autoimmune disease, immune complex consumption
Very low: renal involvement more likely in lupus
Genetic deficiency: recurrent bacterial infections
What Can Cause Abnormal Levels?
Active SLE (lupus)
45% likelyImmune complexes in lupus activate complement, consuming C3 and C4. Low complement = active disease.
Immune Complex Disease
30% likelyPost-infectious glomerulonephritis, cryoglobulinemia, serum sickness all consume complement.
Membranoproliferative Glomerulonephritis
Kidney disease with complement consumption.
Liver Disease (low production)
Liver makes C3. Severe liver disease reduces production.
Inherited Deficiency
Rare. Predisposes to both infections and SLE.
What You Can Do
Low C3 in SLE: discuss with rheumatologist about disease activity
Impact: May need treatment intensification \u00B7 Timeline: Prompt
Monitor C3 and C4 together with anti-dsDNA in lupus
Impact: Falling complement + rising dsDNA = flare \u00B7 Timeline: q3-6 months
If lifestyle changes aren't enough:
If persistently low without known cause: evaluate for kidney disease
Impact: Complement-mediated nephritis can present with low C3 \u00B7 Timeline: As needed
Recommended retest: q3-6 months in SLE; with anti-dsDNA for flare monitoring
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