Relationships between C3b receptor (CR1) activity of erythrocytes and positive Coombs' tests.

Although positive direct Coombs' tests occur in most patients with active systemic lupus erythematosus (SLE), haemolytic anaemic associated with antibody to erythrocytes (E) occurs in less than 10%. Our studies show an association between positive direct Coombs' tests and both the presence of circulating immune complexes and diminished activity of the C3b receptor (CR1) of E. Data presented in this report suggest that in vivo binding of immune complexes and complement by the CR1 of E results in positive direct Coombs' tests in the absence of antibody to E. These observations explain the low frequency of haemolytic anaemia compared with the high frequency of direct positive Coombs' tests in patients with SLE.