LCTA are a heterogenous group of antibodies found in many disease states. The recurring observation is that they are able to modulate immune function in vitro. The exact mechanism of this modulation is unknown, however, alteration of cell surface antigens appears to be important. Recent advances in molecular biological techniques have made the surface antigen, against which LCTA is directed, more accessible. Once the surface antigen is identified, the exact role of LCTA may then be elucidated. Endothelial cell damage is a recurring pathologic finding in many of the diseases discussed. Again, identification of the antigen and its similarity to antigens on various cell types may help explain the immunopathogenesis of the different disease processes.