Receptor for the Fc region of complexed immunoglobulins gamma. Low affinity receptor. Involved in a variety of effector and regulatory functions such as phagocytosis of antigen-antibody complexes from the circulation and modulation of antibody production by B-cells. Isoform IIB1 and isoform IIB1' form caps but fail to mediate endocytosis or phagocytosis. Isoform IIB2 can mediate the endocytosis of soluble immune complexes via clathrin-coated pits. Isoform IIB1 and isoform IIB2 can down-regulate B-cell, T-cell, and mast cell activation when coaggregated to B-cell receptors for AG (BCR), T-cell receptors for AG (TCR), and Fc receptors, respectively. Interacts with FGR (By similarity). Interacts with LYN. Binds the cytoskeleton and is not localized in endocytotic pits. Released as a soluble molecule. Widely expressed by cells of hemopoietic origin. The isoforms are differentially expressed. Isoform IIB1 is preferentially expressed by cells of the lymphoid lineage, isoform IIB2 by cells of the myeloid lineage, and isoform IIB3 is released by macrophages and is present in the serum. Isoform IIB1' is expressed in myeloid and lymphoid cell lines, in normal spleen cells, and in resting or LPS-activated B-cells but is not detected in mesenteric lymph node cells. Contains 1 copy of a cytoplasmic motif that is referred to as the immunoreceptor tyrosine-based inhibitor motif (ITIM). This motif is involved in modulation of cellular responses. The phosphorylated ITIM motif can bind the SH2 domain of several SH2-containing phosphatases. Another tyrosine-containing sequence, more C-terminal, accounts for the ability of isoform IIB2 to trigger the phagocytosis of particulate immuno complexes. Glycosylated. When coaggregated to BCR, isoform IIB1 and isoform IIB1' become tyrosine phosphorylated and bind to the SH2 domains of the protein tyrosine phosphatase PTPC1. Phosphorylated by SRC-type Tyr-kinases such as LYN, BLK, FYN and SYK (By similarity). Ly-17 alloantigenic system involves residues 116 and 161. Ly-17.1 mice are Pro-116 and Glu-161; Ly-17.2 mice are Leu-116 and Leu-161. These polymorphisms do not affect IgG binding.