Acts as an activating receptor inducing cytokine production in mast cells. Can act as a positive regulator of TLR9 signaling in macrophages, leading to enhanced production of pro-inflammatory cytokines. Interacts with FCER1G; the interaction may be indirect. Interacts with TLR9. Highly expressed in bone marrow-derived mast cells and macrophages, peripheral blood monocytes and CD11c+ cells, with weaker expression detected in CD11b cells in bone marrow and peripheral blood. Not detected in B220+ cells in bone marrow or spleen, in Thy-1.2+ or CD3+ cells in peripheral blood, spleen or thymus, or in NK1.1+ cells in spleen (at protein level). Widely expressed in various tissues including heart, liver, spleen, lung, kidney, brain, bone marrow, thymus, axillary lymph node and mesenteric lymph node. Highly expressed in macrophage cell lines J774.1 and RAW 264.7 and in mast cell line MC/9. Weak expression detected in B-lineage cell lines WEHI-231 and A20 and in dendritic cell line DC2.4. Not detected in other myeloid cell lines or T-lineage cell lines. Down-regulated in macrophages by single-stranded CpG oligodeoxynucleotide stimulation. Belongs to the CD300 family.