Detection of a novel human class II HLA antigen.

Genetic, molecular and cellular analyses of the HLA-D region of the major histocompatability complex (MHC) in man have led to the definition of three different products. Two of these, DR and MB (the latter also known as DC (ref. 1) and LB-E (ref. 2)) are defined with serological reagents; the third, known as SB (ref. 3) and PL-3 (ref. 4) is defined with primed lymphocyte typing (PLT) cells. The classical features attributed to HLA-D region encoded (class II) molecules are that they are cell-surface dimers consisting of a structurally conserved alpha-chain noncovalently associated with a polymorphic beta-chain and that they are found primarily on B lymphocytes, some monocyte populations, endothelial and certain other cells. Using these criteria a monoclonal antibody, B7/21, was described as reactive with HLA-DR (ref. 7). We have now re-evaluated B7/21 antibody reactivity using mutant lymphoblastoid cell lines. It appears that this antibody does not react with the molecularly defined D region products described to date but instead, recognizes a class II antigen with distinctive molecular characteristics. We provisionally refer to this antigen as FA.