Abstract
Previous studies of erythrocyte CR1 levels in systemic lupus erythematosus (SLE) and other diseases with in vivo complement activation have led to the conclusion that CR1 levels fall because of loss of CR1 from erythrocytes by proteolysis--predominantly in the liver. In order to measure the existence of proteolysed CR1 remnants on erythrocytes an antibody was raised to a peptide corresponding to the CR1 sequence between the proximal standard consensus repeat (SCR) and the transmembrane segment. This antipeptide antibody recognizes a neo-antigen found on trypsinized erythrocytes which has been demonstrated to represent the 'CR1-stump'. The anti-'CR1-stump' antiserum detects proteolysed CR1 on the ex vivo erythrocytes of a patient with cold haemolytic antibody disease (CHAD). However, higher affinity antibodies will be needed to make anti-CR1-stump a satisfactory diagnostic reagent.