The complement system, a central component of innate immunity, exhibits three pathways of activation classical, alternative, and lectin-mediated. C1, a key component of the classical pathway, is actually a complex of three proteins C1q, C1r, and C1s. C1q is serum glycoprotein of 18-polypeptides chains consisting of three non-identical subunits, A (29 kDa), B (26 kDa), and C (246 aa, 19 kDa) in molar ratio of 1:2:2. C1Q in the plasma is complexed with two proenzymes C1r and two C1s molecule to form the first component of complement (C1). Activation of complement via classical pathway is triggered by binding of globular head of C1q to immune complexes containing IgG (Fc-region) or IgM or to a variety of other activating substances, including C-reactive protein, retrovirus, and mitochondria. Subsequent to C1q binding, c1r and C1s are converted to proteolytic enzymes that are responsible to continuation of activation via the classical pathway. Alternatively, high-affinity autoantibodies directly recognize the collagenous “tail” portion of C1q through the antibody F(ab) antigen- combining sites rather than via the Fc domain. Anti-C1q autoantibodies have been commonly identified in patients with autoimmune diseases such as systemic lupus erythematosus (SLE) and hypocomplementemic urticarial vasculitis. Anti- C1q antibodies preferentially localized in the glomeruli of patients with SLE. Lupus nephritis (LN), the renal disease that accompanies SLE, is present in 25–50% of the cases and is the major cause of morbidity and mortality. Anti-C1q autoantibodies have been suggested to be closely associated with LN. This association is concluded from the correlation between anti-C1q autoantibody positivity and renal involvement , the predictive value of anti-C1q autoantibody titers for flares of nephritis, and the accumulation of anti-C1q autoantibodies in LN kidneys. Conversely, in the absence of anti-C1q autoantibodies, no LN develops. However, no causal relatio
