Studies using both human and murine lymphoid cells have established that the predominant immunoglobulin isotypes on the cell surface of B lymphocytes are IgM and IgD

Studies using both human and murine lymphoid cells have established that the predominant immunoglobulin isotypes on the cell surface of B lymphocytes are IgM and IgD (1-6). Moreover, the majority of B cells express both isotypes simultaneously (2, 3, 79). A minority of B cells bear surface IgG (10, 11). Recently, several investigators have studied the immune function of primed (memory) B cells bearing the different isotypes. Mason (12) found that IgGbut not IgM-bearing cells in the thoracic duct lymph of rats carried immunological memory for the adoptive indirect plaque-forming response to the dinitrophenyl (DNP) hapten. Similar findings were reported by Okumura et al. (13) and Yuan et al.' for murine splenic memory B cells to DNP and sheep red blood cells, respectively. Other investigators have suggested that some memory B cells bear surface IgM. Strober (14) showed that small memory cells found in rat thoracic duct lymph express surface IgG, but that large memory cells present shortly after priming express surface IgM. Abney et al. (15) demonstrated that passaged memory B-cell clones which give rise to IgG2a antibody bear IgM receptors on their cell surface. Yuan et a12 have recently reported that cells giving rise to a secondary IgM response may bear little IgM as compared to cells giving rise to the primary IgM response.