IgG-Fc receptors and the clinical relevance of their polymorphisms.

Receptors for the Fc part of IgG form the bridge between immune complexes, antibodies and blood cells. Besides phagocytes, such as granulocytes and monocytes, also lymphocytes and platelets express members of the large family of IgG-Fc receptors or Fc gamma Rs. Three main classes of leukocyte Fc gamma Rs can be distinguished: Fc gamma RI, Fc gamma RII and Fc gamma RIII. Depending on the type of intracellular domain of the leukocyte Fc gamma Rs, interaction of a cell with an antibody-sensitized particle will induce either an activating signal (leading to phagocytosis, degranulation or toxic oxygen formation) or, in case of Fc gamma RIIb, an inhibiting signal. The inhibiting role of Fc gamma RIIb has long been thought to be only important for B-cell responses, however, recently it has been indicated that Fc gamma RIIb may have a much broader role in immune response, also regulating responses of phagocytes. In general, leukocyte Fc gamma Rs have high affinity for IgG1 and IgG3 subclasses. However, the affinity for the various IgG subclasses is influenced by receptor polymorphisms encoded by single nucleotide polymorphisms of the Fc gamma RIIa and Fc gamma RIIIa and Fc gamma RIIIb encoding genes. Because these subtle Fc gamma R changes determine the level of clearance of immune complexes, associations between Fc gamma R isoforms and disease risks have been investigated and also indicated as discussed in this review. Next to the leukocyte Fc gamma Rs, an IgG transport receptor, FcRn (neonatal Fc receptor) has been described. This receptor seems to be important for placental IgG transport and for IgG plasma level homeostasis. FcRn is expressed by endothelial cells and hepatocytes and functions in these cells as a receptor rescuing IgG from destruction, thereby increasing IgG half life. Also blood cells express FcRn and the exact role of FcRn in these cells is still to determined. Thus, although a lot is known, questions on the exact pattern of expression of the IgG receptors and their role in immune response still remain.