Role of IgD and Tδ Cells in the Regulation of the Humoral Immune Response

Since its discovery by Rowe & Fahey (1965), IgD has not been assigned a clearly defmed function. In fact, the role of IgD in the immune response is generally described in terms of what it does not do, rather than what it does. Thus, it is not secreted following antigenic or mitogenic stimulation of IgD+ B cells, does not appear to neutrahze antigen and does not fix complement. Taken together with the well established fact that IgD is present on the surface of the majority of B lymphocytes, these negative functional criteria have been used to support the view that IgD is primarily a cell-surface antigen receptor (Pemis 1971). The probable importance of IgD in the humoral immune system is underscored by the fact that the forces of evolution have ensured the conservation of the delta (S) heavy chain gene in phylogenetically divergent species (Leslie & Martin 1978). Indeed, it has been suggested that the delta heavy chain gene system originated early in evolution and branched from the alpha chain shortly after divergence of tbe gene systems for alpha and mu (Lin & Putnam 1981, Putnam et al. 1981).