Inhibitory receptor on lymphocytes that negatively regulates antigen receptor signaling via PTPN6/SHP-1 and PTPN11/SHP-2 (PubMed:12796776, PubMed:14652006, PubMed:15568026, PubMed:18193050). May interact in cis (on the same cell) or in trans (on other cells) with TNFRSF14 (PubMed:19915044). In cis interactions, appears to play an immune regulatory role inhibiting in trans interactions in naive T cells to maintain a resting state. In trans interactions, can predominate during adaptive immune response to provide survival signals to effector T cells (PubMed:19915044). {ECO:0000269|PubMed:12796776, ECO:0000269|PubMed:14652006, ECO:0000269|PubMed:15568026, ECO:0000269|PubMed:18193050, ECO:0000269|PubMed:19915044}. This is the function of BTLA (B and T lymphocyte associated, ENSG00000186265).