Differentiation of bone marrow-derived lymphocytes.

Murine lymphocytes were enzymatically radioiodinated and the class of immunoglobulin (isotype) on the cell surface was studied as a function of differentiation and ontogeny. The results indicate that IgM is the first isotype to appear and that IgD is subsequently acquired. A proportion of IgM-bearing cells also bear IgD. The acquisition of IgD does not appear to be under the influence of the thymus or of exposure to antigen. The observations suggest a sequence of differentiation steps in which B-cells first express IgM then acquire IgD (to become "double bearers"), and eventually lose IgM. The relationship of the IgD-bearing cells to the IgG-bearing memory cells has not yet been established. The implications of these findings with regard to function and genetic organization are discussed.