Fc receptor

Fc receptor is a receptoron hematopoietic cellssuch as macrophages, neutrophils and mast cells. They will bind to the constant end of immunoglobulinafter the anti-bodies have binded to antigens. The Fc receptors therefore increase the affinity phagocytic cells have on microbes. This causes phagocytosisand subsequent killing of the pathogen.

There are seven different types of Fc receptors. All of those are stimulating Fc receptors, exept for the inhibiting Fc? RII-B1 and B2. These two receptors contain immunoreceptor tyrosinebased inhibition motifs (ITIMs) in their cytoplasmic tail, which activate inhibitory cellular responses.

Other actions

When IgGmolecules, specific for a certain antigen or surface component, bind to the pathogen with their Fab region (fragment binding region), their Fc regions are pointed outwards, in direct reach of phagocytes. Phagocytes can now bind those Fc regions with their Fc receptors. On contact, a lot of loose bindings are formed, but together this is a tight system. This low individual affinity prevents Fcs from binding Fc receptors in the absent of antigen. This also prevents clotting of IgG on phagocyteswhen there is no antigen.
After the pathogen has been bound, interactions between Fc and Fc receptors result in engulfment of the pathogen. This engulfment is an active proces of binding and releasing of the Fc:Fc receptor complex, in which the cell membrane of the phagocyte gradually encloses the pathogen.

Fc? RI has a different function. As indicated in the table above, its relative binding is much greater than the other ones. Fc? RI is the Fc receptor on granulocytes, essential for acting in allergic reactionsand parasitic infections. In normal situations, in absence of an allergic antigen or parasite, these receptors are loaded with immonoglobulin IgE. In case of a antigen or parasite, it has to at least cross link two IgE molecules and their subsequent receptors on the surface of a mast cell. If the antigen succeeds in doing that, the mast cell is triggered to release its granulesfilled with inflammatory mediators. This method of releasing is pretty fast, since the granules were prepackaged and ready for release.
Pathogens that can not be ingested by phagocytes, like parasites such as Schistosoma mansoni, are covered with IgE and activated eosinophilswill bind them with there Fc? RI and will literally pour its granules onto the parasite.

NK cellsalso have Fc receptors. These cells play an important role in innate immunity, so it is in some paradoxical that they bear Fc receptors, since those are structures that need immunoglobulins: proteins produced during adaptive immunity. And since they are part of innate immunity they need preformed antibody. This is only available in a secondary respons, in which one encounters the same pathogen/antigen again.

This article is licensed under the GNU Free Documentation License.
It uses material from the http://en.wikipedia.org/wiki/Fc+receptor Wikipedia article Fc receptor.