CD84, also known as Ly‑9B and SLAMF5, is a type I transmembrane protein in the SLAM subgroup of the CD2 family. SLAM family proteins regulate multiple aspects of immune system function. CD84 exhibits homophilic binding which is mediated by the N‑terminal Ig‑like domain. Ligation induces tyrosine phosphorylation in the cytoplasmic ITSMs which then recruit the signaling adaptor molecules SAP (SLAM‑associated protein) and EAT‑2 (EWS/Fli1‑activated transcript 2). It is widely expressed among hematopoietic cells including hematopoietic stem cells, myeloid cells (e.g. macrophages, monocytes, dendritic cells, granulocytes, and mast cells), platelets and megakaryocytes, and lymphocytes. Within the T cell lineage, CD84 is expressed on thymocytes, CD4-CD8- cells, single positive CD4 or CD8 cells, NKT cells, and on mouse but not human NK cells. Within the B cell lineage, it is expressed on pro‑and pre‑, mature, marginal zone, and memory B cells as well as plasma cells. CD84 signaling inhibits Fc epsilon RI-induced mast cell activation but enhances platelet activation, LPS‑induced macrophage activation, T cell proliferation and IFN‑gamma production, and the interactions between T cells and B cells that are required for germinal center formation.