The 17G9 antibody reacts with the 47-57 kD ICOS protein, also known as inducible costimulatory molecule, and H4. This protein is homologous to the CD28/CTLA-4 proteins. ICOS is expressed on activated T cells and a subset of thymocytes and can costimulate T cells and induce proliferation. In addition ICOS has been shown to be involved in humoral immune responses (B cell germinal center formation). The ICOS ligand, B7h/B7RP-1 and B7-H2 is constitutively expressed in B cell areas of secondary lymphoid organs and can be induced in other tissues by LPS. ICOS stimulation has been shown to potentiate TCR-mediated IL-4 and IL-10 production and has been proposed to play a role in Th2 cell development. ICOS stimulation has been shown to be involved in airway tolerance and the downregulation of pulmonary inflammation.