Figure 1. IgE and its receptors.
IgE antibody uses two identical light and heavy chains and the constant region has four domains (Cε1-Cε4). The two antigen-binding sites are formed by pairing of the variable region of light and heavy chains. IgE is asymmetrically bent at the Cε2-3 linker and folds on itself with the two Cε2 domains folded back and almost touching the Cε4 domains. IgE interacts with the high affinity IgE receptor FcεRI with the Cε2 and Cε3 domains, and with the low affinity IgE receptor CD23 with the Cε3 and Cε4 domains. Anti-IgE antibodies like Omalizumab binds to the Cε3 domain of IgE and can therefore inhibit the binding of IgE to both FcεRI as well as to CD23.