The FGK45 antibody reacts with mouse CD40, a 40-50 kDa glycoprotein member of the tumor necrosis factor receptor (TNFR) family. CD40 is constitutively expressed by antigen presenting cells, including dendritic cells, B cells, and macrophages. The interaction of CD40 and its ligand CD154, expressed on activated T cells, plays an essential role in mediating a broad range of immune and inflammatory responses including T cell dependent immunoglobulin class switching, germinal center formation, and the development of memory B cells. The FGK45 clone is reported to be functionally useful for both in vitro and in vivo activation of CD40 expressing antigen presenting cells. The FGK45 antibody reacts with mouse CD40, a 40-50 kDa glycoprotein member of the tumor necrosis factor receptor (TNFR) family.