“…Engagement of CD40 on B cells by CD40L or by monoclonal antibody [I 127851 (mAb) results in B cell activation, proliferation, aggregation, and immunoglobulin isotype switching [4,6,[9][10][11][12][13]. Soluble forms of CD40 (sCD40) inhibit T cell-dependent B cell activation and immunoglobulin isotype switching [14].We [15, 161 and others [17-201 have recently reported that a defect in the gene encoding CD40L is responsible for X-linked immunoglobulin deficiency with normal or elevated IgM (X-linked hyper IgM).These patients are unable to synthesize immunoglobulins other than IgM or IgD [21,221, suggesting that interaction between CD40 on B cells and its ligand on activated T cells is critical for immunoglobulin isotype switching. Expression of CD40L in T cells is tightly regulated [23].…”