Quantitation of helper factor activity. II. Inhibition of antibody binding

A sensitive direct assay has been developed to quantitate antigen specific helper factors (ThF). The assay depends on the ability of ThF to block the binding of free antibody to immobilized antigen. It has been used to follow the course of purification of ThF produced in short-term cultures of in vitro primed helper cells and purified by affinity chromatography on antibody and antigen immunoabsorbents. The identity of the ThF was confirmed by assaying its biological activity and by its reactivity with anti-Ia antibody and with a monoclonal anti-ThF.