The human secondary immune response to keyhole limpet haemocyanin

The human secondary immune response to the protein antigen Keyhole limpet haemocyanin (KLH) was studied in thirteen normal adults. For secondary immunization and for measurement of delayed hypersensitivity, 100 μg of KLH was given intradermally to each individual between 81 and 375 days after primary immunization. Immunity was assessed by delayed hypersensitivity skin testing and measurement of antibody formation and KLH stimulated in vitro lymphocyte blastogenesis. Twelve of the thirteen subjects had positive KLH delayed hypersensitivity reactions. Prior to secondary immunization all thirteen had circulating lymphocytes responsive in vitro to KLH and all had circulating haemagglutinating antibody. After secondary immunization there was no further increase in the mean KLH-stimulated blastogenesis while the KLH haemagglutinin titre rose due to increased 7S antibody formation. Primary immunization with small doses of KLH was as effective in inducing immunological memory and a subsequent secondary response as immunization with large doses of the antigen.