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. Can mediate natural killer (NK) cell cytotoxicity dependent on SH2D1A and SH2D1B (By similarity). Increases proliferative responses of activated T-cells and SH2D1A/SAP does not seem be required for this process. Homophilic interactions enhance interferon gamma/IFNG secretion in lymphocytes and induce platelet stimulation via a SH2D1A-dependent pathway. May serve as a marker for hematopoietic progenitor cells (PubMed:11564780, PubMed:12115647, PubMed:12928397, PubMed:12962726, PubMed:16037392) Required for a prolonged T-cell:B-cell contact, optimal T follicular helper function, and germinal center formation. In germinal centers involved in maintaining B-cell tolerance and in preventing autoimmunity (By similarity). In mast cells negatively regulates high affinity immunoglobulin epsilon receptor signaling; independent of SH2D1A and SH2D1B but implicating FES and PTPN6/SHP-1 (PubMed:22068234). In macrophages enhances LPS-induced MAPK phosphorylation and NF-kappaB activation and modulates LPS-induced cytokine secretion; involving ITSM 2 (By similarity). Positively regulates macroautophagy in primary dendritic cells via stabilization of IRF8; inhibits TRIM21-mediated proteasomal degradation of IRF8 (PubMed:29434592). {ECO:0000250|UniProtKB:Q18PI6, ECO:0000269|PubMed:11564780, ECO:0000269|PubMed:12115647, ECO:0000269|PubMed:12928397, ECO:0000269|PubMed:12962726, ECO:0000269|PubMed:16037392, ECO:0000269|PubMed:22068234, ECO:0000269|PubMed:29434592, ECO:0000305}. This is the function of CD84 (CD84 molecule, ENSG00000066294).