There are five classes of mammalian immunoglobulins: IgA, IgD, IgE, IgG, and IgM. In mice, the IgG class is further divided into four subclasses: IgG1, IgG2a/ IgG2c (strain specific), IgG2b, and IgG3.The general immunoglobulin structure is described as two heavy and two light chain (H2L2) polypeptides linked together by disulfide bridges of cysteine residues. Mouse IgE, also known as reaginic antibody, exists in serum as a 4-chain monomer of 185–200 kDa. Like IgM, the IgE monomers contain a fourth constant region domain. Serum concentrations of IgE are reported to be >10 ug/ml. IgE is cyolytic for mast cells, where it binds to the Fc RI receptor and is able to mediate the response of granular and lipid mediators of inflammation, including the PCA reaction. The IgE class switch is induced by IL-4 as part of the Th2 response.