Fc receptors

Fc receptors for immunoglobulin A (IgA) have been recognized functionally for many years. The myeloid receptor, FcαRI, is the most thoroughly characterized. It is structurally related to the FcγRs and Fce{open}RI and also associates with the FcR γ chain dimer. In recent years it has become increasingly apparent that another class of Fc receptor, FcRn, plays important roles in the transport of immunoglobulin G (IgG) across the epithelial layer at mucosal surfaces. FcRn shares only limited homology with other FcR and is distantly related to the major histocompatibility complex (MHC) class I family, dimerizing with β₂-microglobulin, the obligate subunit of all class I molecules. The expression levels of FcαRI on a number of cell types can be upregulated or downregulated by certain cytokines or other stimuli. Increased expression can be observed on monocytes and macrophages on treatment with calcitriol, phorbol myristate acetate (PMA), tumor necrosis factorα (TNFα), interleukin 1β (IL-1β), granulocyte-macrophage colony stimulating factor (GM-CSF), and lipopolysaccharide (LPS), whereas decreased expression is reportedly driven by transforming growth factorβ (TGFβ), IFNγ, suramin, and polymeric IgA (pIgA).