Figure 1

The FcεRI and its dose-response behavior. (A) The murine FcεRI consists of an α-, a β-, and two disulfide-bridged γ-subunits. The α-subunit contains only a short cytoplasmic region and binds to the constant Cε3 region of IgE via its second extracellular immunoglobulin-like domain (oval shape). The β-subunit belongs to the tetraspanin family and the γ-subunits can be called small transmembrane adaptor proteins. Due to its association with further activating Fc receptors the γ-subunit is now called FcRgamma. β-subunit and FcRgamma contain ITAMs in their cytoplasmic regions. The ITAMs are phosphorylated by Lyn (visualized by red ovals) subsequent to receptor crosslinking by multivalent Ag. Lyn has been shown to be pre-associated with the FcεRI. PM, plasma membrane. (B) Stimulation of growth factor receptors with increasing amounts of respective ligands usually results in a saturation of the response at high ligand concentrations. (C) The dose–response curve of the FcεRI traverses through three distinct phases. At low Ag concentrations, i. e. a high Ab/Ag ratio, only a weak response can be measured, at medium Ag concentrations (the zone of equivalence between Ab and Ag) an optimal response can be observed, and at high Ag concentrations, i. e. a low Ab/Ag ratio, only a weak response can again be measured. Therefore, this dose–response curve is called bell-shaped.