The G28.5 antibody reacts with human CD40, a 45-48 kDa glycoprotein that is a member of the TNFReceptor superfamily. CD40 is expressed on B cells, follicular dendritic cells, macrophages, endothelial cells, and fibroblasts. CD40 functions as a receptor for CD154 (CD40 ligand) and mediates a wide variety of immune and inflammatory responses including co-stimulation, B cell differentiation, isotype class switching, and germinal center formation.