Self-ligand receptor of the signaling lymphocytic activation molecule (SLAM) family. SLAM receptors triggered by homo- or heterotypic cell-cell interactions are modulating the activation and differentiation of a wide variety of immune cells and thus are involved in the regulation and interconnection of both innate and adaptive immune response. Activities are controlled by presence or absence of small cytoplasmic adapter proteins, SH2D1A/SAP and/or SH2D1B/EAT-2 (PubMed:19648922). Triggers cytolytic activity only in natural killer cells (NK) expressing high surface densities of natural cytotoxicity receptors (By similarity). Positive signaling in NK cells implicates phosphorylation of VAV1. NK cell activation seems to depend on SH2D1B and not on SH2D1A (By similarity). In conjunction with SLAMF1 controls the transition between positive selection and the subsequent expansion and differentiation of the thymocytic natural killer T (NKT) cell lineage (PubMed:18031695). Promotes T cell differentiation into a helper T-cell Th17 phenotype leading to increased IL-17 secretion; the costimulatory activity requires SH2D1A (By similarity). Promotes recruitment of RORC to the IL-17 promoter (By similarity). In conjunction with SLAMF1 and CD84/SLAMF5 may be a negative regulator of the humoral immune response (PubMed:25926831). In the absence of SH2D1A/SAP can transmit negative signals to CD4(+) T-cells and NKT cells. Negatively regulates germinal center formation by inhibiting T-cell:B-cell adhesion; the function probably implicates increased association with PTPN6/SHP-1 via ITSMs in absence of SH2D1A/SAP (PubMed:22683125). However, reported to mediated T-cell adhesion, to participate in stable T-cell:B-cell interactions and to be involved in maintaining B-cell tolerance in germinal centers and in preventing autoimmunity (PubMed:20153220, PubMed:25801429). Involved in regulation of autoimmunity. Isoform 3 may be suppressor of pathogenic T-cell proliferation (PubMed:21422172).