[6] In contrast, G0 glycans, which lack galactose and terminate instead with GlcNAc moieties, have increased affinity for FcγRIIIA.

[6] Another FcR is expressed on multiple cell types and is similar in structure to MHC class I.

[8][9] Recently, research suggested that this receptor plays a role in the homeostasis of IgG serum levels.

[10] It is composed of two extracellular Ig-like domains, and is a member of both the immunoglobulin superfamily and the multi-chain immune recognition receptor (MIRR) family.

For example, macrophages begin to ingest and kill an IgG-coated pathogen by phagocytosis following engagement of their Fcγ receptors.

FcεRI is the Fc receptor on granulocytes, that is involved in allergic reactions and defense against parasitic infections.

When an appropriate allergic antigen or parasite is present, the cross-linking of at least two IgE molecules and their Fc receptors on the surface of a granulocyte will trigger the cell to rapidly release preformed mediators from its granules.

[27] An ITAM is a specific sequence of amino acids (YXXL) occurring twice in close succession in the intracellular tail of a receptor.

When phosphate groups are added to the tyrosine (Y) residue of the ITAM by membrane-anchored enzymes of the Src kinase family, a signaling cascade is generated within the cell.

This adaptor protein is called the Fcγ subunit and, like FcγRIIA, contains the two YXXL sequences that are characteristic of an ITAM.

:[30] Experiments using B cell deletion mutants and dominant-negative enzymes have firmly established an important role for SH2-domain-containing inositol 5-phosphatase (SHIP) in negative signaling.

Negative signaling through SHIP appears to inhibit the Ras pathway through SH2 domain competition with Grb2 and Shc and may involve consumption of intracellular lipid mediators that act as allosteric enzyme activators or that promote entry of extracellular Ca2+.

[24] Many low affinity interactions are formed between receptor and antibody that work together to tightly bind the antibody-coated microbe.

Activation of mast cells following engagement of FcεRI results in a process called degranulation, whereby the mast cell releases preformed molecules from its cytoplasmic granules; these are a mixture of compounds including histamine, proteoglycans, and serine proteases.

They also have an external structure called an integument that is resistant to attack by substances released by macrophages and mast cells.

Activated eosinophils release preformed mediators such as major basic protein, and enzymes such as peroxidase, against which helminths are not resistant.

[38][39] The interaction of the FcεRII receptor with the Fc portion of helminth bound IgE causes the eosinophil to release these molecules in a mechanism similar to that of the NK cell during ADCC.

Earlier studies summarized by Sanders and Lynch in 1993 suggested critical roles for FcRs in CD4+ T cell mediated immune responses and proposed the formation of a joint signaling complex among FcRs and TCR on the cell surface.

[48] This established the current paradigm that T cells do not express FcRs and these findings were never challenged and experimentally tested.

[49] Chauhan and coworkers showed binding of immune complexes (ICs), the FcR ligand to activated CD4+ T cells.

CD16a signaling upregulate the expression of nucleic acid sensing toll-like receptors and relocate them to cell surface.