A Modification of the Hemolytic Plaque Assay for Use with Protein Antigens

A method is presented which allows the in vitro enumeration of cells producing antibody against a variety of protein antigens. The proteins are covalently linked to red blood cells by means of a carbodiimide reagent. The concentrations of protein required for the plaque assay are greater than those required for passive agglutination. The method is simple and sensitive and the results mimic the kinetics of the response that is seen in in vivo assays of serum antibody. Both direct (probably 19 S) and indirect (probably 7 S) plaque-producing cells are detected.