Antibodies specific for human albumin function as blocking antibodies when attached to erythrocyte-bound albumin.

Albumin is shown to be firmly bound to human red blood cells using the techniques of flow cytometry, immunoblotting, and complement fixation. The interactions between antibodies attached to the cell bound albumin and the complement system are examined. Antibodies specific for human serum albumin bind to albumin on erythrocytes and activate both homologous and heterologous complement in the absence of hemolysis. Moreover, treatment of erythrocytes with anti-albumin antibodies renders the cells resistant to classical pathway mediated lysis initiated by a passive lysis system. Thus, erythrocyte-bound anti-albumin antibodies appear to function in a manner similar to "blocking" antibodies described in some bactericidal systems.