The FGK4.5 monoclonal antibody reacts with mouse CD40 also known as Bp50. CD40 is a 48 kDa type I transmembrane glycoprotein that belongs to the tumor necrosis factor receptor (TNFR) superfamily. CD40 is expressed broadly on antigen-presenting cells (APCs) such as dendritic cells, B cells, macrophages, and monocytes as well as non-immune endothelial cells, basal epithelial cells, and a range of tumors. Upon binding to its ligand CD154, CD40 acts as a costimulatory molecule for the activation of B cells, dendritic cells, monocytes, and other APCs. CD40 plays roles in B cell activation, differentiation, proliferation and Ig isotype switching as well as dendritic cell maturation. Agonistic CD40 monoclonal antibodies have been shown to activate APCs and promote anti-tumor T cell responses. The FGK4.5 antibody is an agonistic antibody that has been shown to activate CD40 expressing APCs. FGK4.5 can also be used to inhibit CD40/CD154 interaction in vitro and in vivo.