Immune Complexes, C1q Binding

The Immune Complexes, C1q Binding test (also known as the C1q binding test or circulating immune complexes, CIC) is used to evaluate and monitor immune complex–mediated, type III hypersensitivity disorders, including systemic lupus erythematosus, glomerulonephritis, vasculitis, rheumatoid arthritis, and related autoimmune diseases. It assists in assessing immune complex activity in chronic, persistent infections and in following conditions associated with complement deficiency. Results may be trended to gauge response to therapy and, in selected malignancies where immune complexes are implicated, to aid in disease monitoring.

Circulating immune complexes arise when soluble antigens bind antibodies in the bloodstream. Under normal conditions they are removed by the mononuclear phagocyte system, with larger complexes cleared predominantly by the spleen and liver. When antigen is in excess, complex formation is increased, or clearance is impaired, smaller complexes may persist, deposit in tissues, activate complement, generate C3a and C5a, recruit inflammatory cells, and produce tissue injury. Preferred sites of deposition include vascular endothelium, renal glomeruli, and synovium, leading to clinical syndromes such as vasculitis, glomerulonephritis, and arthritis. Persistent infections and serum sickness–type reactions can elevate levels of circulating complexes. The C1q-binding assay detects complexes that bind C1q and reflects systemic immune complex activity; it does not quantify complexes already deposited in tissues. Results should be interpreted alongside the clinical picture and other laboratory findings.