Coevolution of Mucosal Immunoglobulins and the Polymeric Immunoglobulin Receptor: Evidence That the Commensal Microbiota Provided the Driving Force

Figure 3

Fc receptors are Ig isotype- and cell type-specific. These schematic diagrams illustrate basic structural features of selected Fc receptors, all of which are Type I transmembrane proteins. The structures are not to scale and do not imply specific 3-dimensional conformations. All of the structures represent the human homolog of the indicated protein except for FcRY, which is from chicken. Binding of the constant (Fc) region of Igs to cellular receptors initiates transport across epithelial cells or intracellular signaling in cells of hematopoietic origin. The membrane-proximal Ig-like domain of FcRn pairs with the Ig-like domain of soluble β2-microglobulin (β2m). The Ig-binding subunits of FcαRI, FcγRI, FcγRII, FcγRIII, and FcεRI (shown here) pair with other protein subunits (not shown) to initiate signaling pathways. This figure was compiled using information from [9–15].