The 2F1 monoclonal antibody specifically binds to KLRG1 (Killer cell Lectin-like Receptor G1), which is the mouse homolog of the rat mast cell function-associated antigen (MAFA), on all mouse strains tested (eg, AKR/J, BALB/c, C3H/HeN, C3H.SW, C57BL/6, DBA/1, SJL, 129/J). Unlike rat MAFA, which is expressed on mast cells, mouse KLRG1 is expressed on a large subset of NK cells, lymphokine-activated killer (LAK) cells, adherent LAK (A-LAK) cells, subsets of activated CD8+ T lymphocytes, and small fractions of CD4+ and CD8+ T cells, but not mast cells. The expression of KLRG1 is correlated with reduced proliferative capacity of activated T lymphocytes or reduced effector functions of activated NK cells. KLRG1 plays a role in the regulation of leucocytes of both the innate and adaptive immune system. The 2F1 mAb reportedly stains the rat basophilic leukemia cell line, RBL-2H3, which is known to express MAFA. The KLRG1 protein is an inhibitory lectin-like type II transmembrane receptor containing a cytoplasmic motif similar to ITIM (Immunoreceptor Tyrosine-based Inhibitory Motif); its ligand has not been identified KLRG1 is expressed mainly as a homodimeric molecule consisting of two N-glycosylated subunits of approximately 30-38 kDa. The level of KLRG1 expression is reduced in MHC class I-deficient mice, although direct binding of KLRG1 to MHC class I antigens could not be detected. Crosslinking of KLRG1 by 2F1 mAb reduces TCR-mediated Ca++ mobilization and cytotoxic responses (but not IFN-γ production) by CD8+ T cells and inhibits IFN-γ and TNF production and redirected lysis by NK cells.