Antibodies have a common basic structure: each consists of two heavy and two light identical polypeptide chains, linked via disulfide bridges. The heavy and light chains include constant and variable regions. The variable regions of both chains determine the binding domain (paratope) for a specific antigenic determinant (epitope). The epitope is the part of the antigen that is recognized by the antibody. In addition, the constant regions of the heavy chains determine the immunoglobulin class to which the antibody belongs (IgM, IgG, IgD, IgA or IgE). The immunoglobulin class is also called isotype. The various antibody classes are found in different compartments of the body. For example, IgA is present in the saliva while IgG and IgM are found in the blood. In addition, membrane-bound antibodies are also found (e.g.: IgE on mast cells or IgD on B lymphocytes).