Complement 1q

Complement 1 (C1) is part of the C1 complex, which comprises three subunits, C1q, C1r, C1s, and is the first component of the serum complement system. C1q recognizes and binds to immunoglobulin complexed to antigen and initiates the complement cascade. C1 deficiency is associated with an increased incidence of immune complex disease (systemic lupus erythematosus, polymyositis, glomerulonephritis, and Henoch-Schonlein purpura). Low C1 levels have also been reported in patients with abnormal immunoglobulin levels (Bruton and common variable hypogammaglobulinemia and severe combined immunodeficiency). Anti-C1q anti-serum assays are used to quantitate C1q antigen levels.