IgG4 binding factor in human seminal plasma identified as heavy chain of immunoglobulin

Human seminal plasma contains a factor that binds human immunoglobulin G (IgG). The factor has an estimated M(r) of 50 kD and interacts specifically with human IgG4. It does not bind other subclasses of human IgG or IgGs of other mammalian species tested. The factor was purified by affinity chromatography on protein G column. The 50-kD component was eluted in the adsorbed fraction and immunostained with monoclonal antibodies against heavy chain (gamma) of IgG. Purified subclasses of human serum IgG were separated into heavy and light chains by sodium dodecyl sulfate polyacrylamide gel electrophoresis under reduced condition. The heavy chains of all subclasses of IgG bound IgG4. The present findings suggest that the 50-kD IgG4 binding factor of human seminal plasma is the heavy chain of IgG.