Complement C3

Complement C3 testing (also known as C3 complement or Beta-1C globulin) is used to document congenital deficiency of this component and to assess consumption of complement in immune disorders. It supports evaluation of activation of the classical versus alternative pathways when interpreted with C4, aids diagnosis of conditions linked to complement protein deficiency, and helps track autoimmune disease activity and response to therapy—concentrations commonly decline as disease activity increases. Clinically, C3 measurement is applied in systemic lupus erythematosus, chronic active hepatitis, selected chronic infections, and glomerulonephritides, and it contributes to assessment of immune status in infectious diseases.

The complement system consists of plasma proteins (C1–C9) that amplify host defense, promote opsonization, facilitate phagocytosis, and induce cytolysis. C3 constitutes the largest fraction of complement proteins and functions within both the classical pathway, initiated by antigen–IgG/IgM immune complexes, and the alternative pathway, which can be triggered by polysaccharides, endotoxins, IgA/IgE, and immunoglobulin fragments. Deposition of C3 fragments on microbial surfaces enhances recognition and uptake by phagocytes and contributes to leukocyte recruitment and degranulation. Lower C3 concentrations typically indicate increased consumption during immune complex–mediated inflammation and parallel activity in selected autoimmune and glomerular diseases. The assay measures total C3, capturing both biologically active and inactive forms.