Multiple heavy chain isotypes on the membrane of the small B lymphocytes in human blood

Small lymphocytes from adult human blood were examined for the presence of membrane-associated alpha, gamma, delta and mu Ig isotypes by means of a direct immunofluorescence technique. Since less than 10% of the small lymphocytes in blood are B cells as defined by positive reactivity with an anti-Fab conjugate, our experiments were performed on a T cell-depleted fraction in which about 80% of the small lymphocytes were B cells. With the two-wavelength immunofluorescence method, all of the double-isotype combinations were found. The percentage of cells bearing more than two isotypes was deduced. The delta mu and the alpha delta mu combinations were the most common, as previously found on tonsillar lymphocytes. In contrast with the tonsils, no lymphocytes bearing only delta were observed and the proportions of alpha gamma- and alpha gamma mu-bearing lymphocytes were very small. The presence of lymphocytes bearing four isotypes could practically be excluded.