Fc epsilon receptor

The Fc epsilon receptor (Fcε receptor - see below Fc receptor) binds the Fc domain of free IgE. Fcε-receptors are found on the surface of mast cells and basophilic granulocytes. Binding of antigens to these bound antibodies results in activation of the respective cell. The IgE/Fc-RI interaction is followed by antigenic cross-linking of receptors at the plasma membrane level. The chains of Fc-RI do not aggregate without antigen contact. This is possibly prevented as a consequence of the extensive glycosylation of the front and back sides of the molecule.

These processes play a crucial role in type I allergy.

In mammals, two types of Fcε receptors are distinguished,

• FcεRI, receptors with high affinity to IgE.
• and
• FcεRII, receptors with low affinity to IgE.

The high-affinity IgE receptor (FcεRI) can bind monomeric IgE. The low-affinity IgE receptor FcεRII(CD23) interacts preferentially with IgE complexes.

Fc epsilon receptor I

High affinity Fc epsilon receptor I (FcεRI) is produced by eosinophil granulocytes mast cells and basophil granulocytes, but also by monocytes/macrophages and dendritic cells. The Fc epsilon receptor I is involved in allergic reactions and in the defense against parasitic infections.

The FcεRI on mast cells and basophilic granulocytes consists of an alpha and beta chain as well as 2 gamma chains. In contrast, monocytes/macrophages, eosinophilic granulocytes and dendritic cells produce trimeric receptors consisting of one alpha and 2 beta chains.

IgE binding occurs through the alpha chain of the receptor. FcεRI does not bind immunoglobulins other than IgE. A receptor can bind one monomeric IgE molecule at a time. Its affinity for IgE is very high. The half-life of receptor binding to IgE is several days. After that, the IgE molecule is released again and is available for re-binding. On mast cells, binding to FcεRI leads, among other things, to an increase in FcεRI expression.

Under physiological conditions, a mast cell expresses between several thousand and one million high-affinity IgE receptors. However, degranulation of the mast cell does not occur by simple binding. This occurs only by cross-linking of at least 2 FcεRI. Here, the strength of mast cell degranulation depends on the number of cross-linked FcεRI.

IgE-induced activation and degranulation does not usually cause the death of mast cells. Thus, mast cells can survive multiple cycles of de- and regranulation. After IgE-induced degranulation of the mast cell, a refractory phase occurs. In the skin, this refractory phase, during which the mast cell can show no, or only a very weakened immune response, lasts only a few days. Afterwards it is fully functional again. The beta-chain and the gamma-chains of the FcεRI have functions in the signal transduction after the IgE activation of the receptor.

Fc epsilon receptor II (FcεR II).

Most low-affinity Fc epsilon receptors (FcεR II) are expressed by B lymphocytes. FcεR II has multiple functions as a membrane-bound or even soluble receptor. Fc epsilon Receptor II controls the growth and differentiation of B cells. Furthermore, FcεR II blocks IgE binding of eosinophils, monocytes and basophils.