Complement Component C1q and Anti‐C1q Antibodies in Theory and in Clinical Practice

The complement system is a major part of the innate immunity. The first component of the classical pathway of complement activation, C1q, plays a crucial role in the clearance of immune complexes and apoptotic bodies from the organism. Autoantibodies against C1q (anti-C1q) have been found in a number of autoimmune and infectious diseases. They have been best described in patients with systemic lupus erythematosus, where they are thought to play a pathogenic role in lupus nephritis (LN). Their high negative predictive value for the occurrence of active proliferative LN, as well as their possible ability to indicate a renal flare as soon as 6 months in advance, have rendered anti-C1q antibodies a novel non-invasive tool in the detection of active LN.