The expression of surface IgD on B cells responsive to thymus-independent and thymus-dependent antigens and its requirement for B-cell triggering.

Cells separated by the fluorescence activated cell sorter on the basis of their surface IgD (sIgD) phenotype have been examined for responsiveness to thymus-dependent and thymus-independent antigens. The ability of monoclonal anti-IgD alloantibodies to inhibit responses in vitro to the various classes of antigen has also been investigated. Evidence is presented indicating that both sIgD positive and sIgD negative cells can respond to all types of antigen tested. However, although the presence of sIgD was necessary for the response of sIgD positive cells to thymus-dependent antigens, the presence of this isotype was not obligatory for the response of the sIgD positive population to thymus independent antigens. The possible role of sIgD as the obligatory purveyor of a B-cell activation signal is discussed in the light of these findings.