Primary and secondary immune responses The primary immune response occurs on initial contact with a foreign antigen and results in production of immunoglobulin M and then IgG. If the same antigen is encountered again, immunological memory leads to a secondary immune sponse that produces a much more rapid and larger increase in specific IgG productionPrimary and secondary immune responses • The primary immune response occurs on initial contact with a foreign antigen and results in production of immunoglobulin M and then IgG. • If the same antigen is encountered again,immunological memory leads to a secondary immune rsponse that produces a much more rapid and larger increase in specific IgG production