Candidate for immunoglobulin D present on murine B lymphocytes

ALTHOUGH the original discovery of human IgD is almost ten years old 1 , it is only very recently that a possible function for this class of immunoglobulin has been suggested. Present in normal human serum in very small amounts 2 , IgD is found on the surface membrane of relatively large numbers of human peripheral lymphocytes, in particular those obtained from cord blood and the circulation of patients with chronic lymphatic leukaemia 3–7 . IgD is frequently associated with IgM on the same cell 5–7 and this, together with the fact that the frequency of IgD-bearing lymphocytes is higher in cord blood than adult blood 4,5 , suggests a fundamental role for IgD, either as a primitive recognition unit or for regulation of the immune response. Were this to be so, then clearly IgD would be expected to occur in species other than man. Here we present the identification of an immunoglobulin present on the surface of mouse splenic lymphocytes which is not IgM, but which has the predicted characteristics of IgD.