Immunoglobulins are produced by B cells (antibodies) in response to an immunogen and are among the most abundant proteins in the blood, comprising 20% (by weight) of total plasma proteins. The immunoglobulins derive their name from the fact that when antibody-containing serum is subjected to electrophoresis, five main peaks of proteins are identified based on their differential ability to migrate in an electric field. The various antibodies produced by plasma cells are classified by isotype, each of which differs in function and antigen responses primarily due to structure variability. Five major antibody classes have been identified in placental mammals: IgA, IgD, IgE, IgG and IgM. This classification is based on differences in amino acid sequence in the constant region (Fc) of the antibody heavy chains. IgG and IgA are further grouped into subclasses (e.g., in human IgG1, IgG2, IgG3, IgG4, IgA1 and IgA2) based on additional small differences in the amino acid heavy chain sequences.