Complement 3 (C3)

A complement C3 blood test measures the level of the complement component 3 protein, part of the immune system’s complement pathway. It helps evaluate immune function and inflammation, detect complement deficiencies, and support diagnosis or monitoring of autoimmune diseases (e.g., lupus), recurrent infections, and kidney disorders caused by complement activation. Low levels often indicate complement consumption in active immune‑complex disease; high levels reflect acute inflammation.

In systemic lupus erythematosus (SLE), C3 is typically low during active disease because immune-complex formation activates and consumes complement. Falling C3 levels often signal a flare—particularly lupus nephritis—while normal or rising C3 suggests remission. However, C3 can be affected by other conditions (infections, genetic deficiencies), so trends and clinical context guide interpretation.

C3d is a cleavage fragment of complement C3 that tags pathogens and immune complexes, functioning as an opsonin and linking innate to adaptive immunity. It binds complement receptor 2 (CR2, CD21) on B cells, lowering their activation threshold and enhancing antibody responses. C3d also promotes phagocytosis and efficient clearance of immune complexes, aiding antigen presentation and immune regulation.

Elevated serum C3 or complement activation has been reported in several solid tumors, including hepatocellular carcinoma, ovarian and other gynecologic cancers, non–small cell lung cancer, renal cell carcinoma, and some gastrointestinal cancers (colorectal, gastric). Increases often reflect tumor secretion or systemic inflammation and vary with stage; results require clinical correlation and specialist interpretation.

Anxiety and chronic psychological stress can activate inflammatory pathways and may modestly raise complement C3 indirectly. However, markedly elevated C3 is more commonly linked to infections, autoimmune disease, metabolic syndrome, obesity or liver disorders. Anxiety alone is an uncommon sole cause. If your C3 is high, discuss further evaluation with a healthcare provider to rule out these conditions.

Complement C3 deficiency causes recurrent, often severe pyogenic infections—meningitis, sepsis, pneumonia, otitis media, sinusitis, skin infections and abscesses—especially from encapsulated bacteria. It can present early in life with failure to thrive and poor wound healing. Immune-complex–mediated complications such as membranoproliferative (C3) glomerulonephritis may cause hematuria, proteinuria and renal impairment. Recurrent sinus and lung infections may lead to bronchiectasis.