A plaque assay for all cells secreting Ig of a given type or class

Abstract A modification of the hemolytic plaque assay using protein A‐coated red cells is described which makes use of the fact that the Fc portion of IgG binds to protein A. A number of murine plasmacytomas secreting different classes of Ig have been tested for plaque formation with these indicator red cells. In the presence of complement‐binding antibodies specific for the corresponding class of secreted Ig, between 10 and 70 % of all plated myeloma cells formed plaques. The assay shows a prozone effect in excess of antibody, suggesting that complexes of antibody and secreted Ig effect lysis of the target cells. This assay can be used to enumerate cells secreting any molecules for which complement‐binding antibodies are available.