Acts as a MHC class II receptor (PubMed:20519654). When stimulated on its own, does not play a role in cytokine production or the release of cytotoxic granules by NK cells and cytotoxic CD8(+) T cells (PubMed:17213291, PubMed:18991291). Does not act as an Fc receptor (PubMed:18991291). Interacts (tyrosine phosphorylated) with PTPN11 (PubMed:17213291, PubMed:20933011). Interacts (tyrosine phosphorylated) with PTPN6, INPP5D, INPPL1 and GRB2 (PubMed:20933011). Interacts with class II MHC HLA-DR when the alpha chain is associated with a beta-1, beta-4 or a beta-5 but not a beta-3 chain (PubMed:20519654). Expressed by cytolytic cells including NK cells, effector and effector-memory CD8(+) T-cells, and a subset of NKT cells (at protein level) (PubMed:17213291, PubMed:18991291, PubMed:20933011). Also expressed in gamma delta T cells and in a rare subset of effector CD4(+) T-cells (at protein level) (PubMed:18991291). Expressed in spleen, skin, peripheral blood leukocytes, liver, lung, bone marrow, small intestine and placenta (PubMed:20933011). Expression among T-cells is greatly expanded in HIV-1 infected individuals, and includes not only effector and effector-memory CD8(+) T-cells but also populations of CD4(+) T-cells (PubMed:17213291, PubMed:20933011). Expression among CD8(+) T-cells and NK cells is expanded in individuals with chronic lymphocytic leukemia (CLL) but is reduced in PBMCs from patients with acute (AML), chronic myeloid leukemia (CML) and non-Hodgkin's lymphoma (PubMed:18991291, PubMed:20933011). Expression is higher in PBMCs and/or CD3(+) cells of patients with autoimmune diseases, such as rheumatoid arthritis (RA), systemic lupus erythematosus (SLE) and idiopathic thrombocytopenia purpura (ITP) (PubMed:20933011). In contrast, expression in CD3(+) cells from patients with lupus anticoagulans (LA) is higher (PubMed:20933011). Down-regulated upon stimulation with mitogen phytohaemagglutinin (PHA) or concavalin A in peripheral blood mononuclear cells (PBMCs). Contains 1 copy of a cytoplasmic motif that is referred to as the immunoreceptor tyrosine-based inhibitor motif (ITIM). The phosphorylated ITIM motif is involved in PTPN11 binding. Phosphorylated on Tyr residues. Tyrosine phosphorylation induces association with phosphatase PTPN11, PTPN6, INPP5D, INPPL1 and GRB2.