The HM40-3 monoclonal antibody reacts with mouse CD40, a 48kDA type I glycoprotein known as Bp50. CD40 is part of the TNFR family and expressed on a subset of T cells, B cells, dendritic cells, and macrophages. It has been shown to be involved in the protection of B cells from apoptosis, B cell differentiation, co-stimulation, and isotype class-switching. The HM40-3 antibody blocks the binding of CD40L (CD154) and stimulates B and dendritic cell proliferation.