Binding of one monoclonal antibody to human Ia molecules can be enhanced by a second monoclonal antibody.

A monoclonal anti-human Ia antibody, PTF 29, was tested for its ability to bind purified, 125I-labeled human Ia preparations. It was found that the binding level increases considerably in the presence of a second monoclonal antibody. Experimental conditions were selected under which only the binding of PTF 29 and not the binding of the second antibody could be determined. Under these conditions, it was found that "helped" PTF 29 binding has higher affinity and is exerted on a molecular subset different from that bound by PTF 29 alone. This phenomenon, while not easily accommodated in the present conceptual framework of the human Ia system, appears in itself interesting and may have more general implications.