The 27A12 monoclonal antibody reacts with mouse ICOS (inducible T cell co-stimulator). ICOS is a 47-57 kDa homodimeric glycoprotein belonging to the CD28 family of costimulatory molecules. ICOS is expressed on activated T cells and upon ICOSL binding, co-stimulates T and B cell responses. The ligand Is expressed on antigen presenting cells including splenic B cells, dendritic cells, and macrophages. ICOS signaling is also thought to be important for maintaining regulatory T cell homeostasis. The 27A12 antibody is useful for identifying ICOS expressing cells via flow cytometry while the 7E.17G9 anti-ICOS antibody has been shown to block the binding of ICOSL to ICOS in vivo.