TheC1componentofcomplementisacomplexof2C1qsub-unitsandatetramercomprising2C1rand2C1ssub-units.C1activationisregulatedbyaninhibitor(C1inhibitor).ReductionsinthelevelsofcomponentsC3,C4,C1andC2canbeassociatedwithglomerulardisease,endocarditisandsystemiclupuserythematosus(SLE).C1qhasamolecularweightof410kDa.