Fc-receptor(s)

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Introduction

Fc-receptor(s) are structures on the surface of cells and can contribute to the protective functions of the immune system. The name of Fc-receptors is based on binding and specificty for their counter-part or ligand, namely the Fc-portion of an antibody. Functionally, Fc-receptors can be involved in antibody-mediated phagocytosis and antibody-dependent cell-mediated cytotoxicity. Structurally, Fc-receptors are classified based on the type of antibody recognized

Functions of Fc-receptors

Fc-receptors can be found on cells of the immune system like macrophages, monocytes, neutrophils, Natural killer cells and others. Cells with Fc-receptors can bind antibodies at the Fc-region of an antibody. The binding ability between the Fc-receptors and the Fc-part of antibodies allows finding and binding of antibody covered objects. Thus, if the immune system works effectively and protectivey, cells with Fc-receptors like macrophages can bind to sickening bacteria or other microbial pathogens which are covered with antibodies via an interaction between Fc-receptors and antibodies. Once bound to the macrophages by this mechanism, the macrophage can start to ingest ^* the microbial pathogen in a process called phagocytosis and kill it. In this scenario the Fc-receptor contributes to protective immunity. Another function of Fc-receptors occurrs in a process called antibody-dependent cell-mediated cytotoxicity (ADCC). Under certain circumstances, Fc-receptors on the surface of Natural killer cells allow to kill antibody covered target cells. The consequence of the Fc-receptor antibody interaction is not eating or phagocytosis of the covered target inside the immune cells like the macrophage, but killing the target from the outside by the bystander Natural killer cell.

Classes of Fc-receptors

Fc-receptors are classified according to the type of antibody or immunoglobulin (Ig) recognized:

• Fc-alpha receptor (FcαR) binds immunoglobulin A (IgA)
• Fc-gamma R (FcγR) binds immunoglobulin G (IgG). These receptors can be subdivided according to antibody affinity and molecular structure: FcγRI (Fc-gamma receptor 1) has a high affinity compared with FcγRII (Fc-gamma receptor 2) and FcγRIII. Also, FcγRI has three extracellular immunoglobulin-like domains compared to 2 domains FcγRII and FcγRIII in mice or man.
• Fc-epsilon receptor (FcεR) binds immunoglobulin E (IgE), and is similarly differentiated by affinity into FcαRI and FcαRII.
• Neonatal Fc-receptors (FcRn) are an atypical class as they are termed after their occurrence at a developmental stage, but not their antibody specificity. FcRn were discovered at the developmental stage of newly born individuals i.e. neonatal individuals and these receptors bind IgG under the right circumstances.

References

Immunobiology. 5th ed.Janeway, Charles A.; Travers, Paul; Walport, Mark; Shlomchik, Mark. New York and London: Garland Publishing; c2001.

Cellular and molecular immunology / Abul K. Abbas, Andrew H. Lichtman ; illustrations by David L. Baker, Alexandra Baker. Philadelphia, PA : Elsevier Saunders, c2005

Stimulatory and inhibitory signals originatingfrom the macrophage Fcγ receptors.Jeffrey S. Gerber and David M. Mosser. Microbes Infect. 2001 Feb;3(2):131-9.