The 6A8.6F10.1A6 monoclonal antibody reacts with the Qa-1[b] alloantigen, which is a nonclassical MHC class I (Class Ib) molecule encoded by the T23 gene of the H-2 complex (H2-T23). Qa-1 associates with β2-microglobulin and is expressed at low levels on most leukocytes and many other cell types. Its level of cell-surface expression is upregulated by IFNγ or specific peptides. Qa-1 is an oligomorphic molecule which presents a limited pool of peptides to T lymphocytes bearing αβ and γδTCR and binds to a large subpopulation of NK cells. In fact, Qa-1[b] is the ligand for CD94/NKG2A, CD94/NKG2C, and CD94/NKG2E receptors, which are expressed on NK cells. Furthermore, it has been reported that Qa-1 expressed on activated B lymphocytes is involved in immunoregulation by inducing T-cell-mediated suppression of antibody responses. The 6A8.6F10.1A6 mAb can detect Qa-1[b] on activated splenocytes from C57BL/6 and BALB/c mice (both Qa-1[b]), but not from A/j mice (Qa-1[a]), and it can block target cell recognition by CTLs specific for Qa-1-presented antigen.