Antibodies, also referred to as immunoglobins, are made by B lymphocytes and are an essential part of humoral immunity. There are five general classes of antibodies and each of the large number of antibody molecules responds to a specific foreign molecule called an antigen.
The variable (V) region is the portion of the antibody that distinguishes it from all others. This region is where the antigen binds to the antibody. The remainder of the antibody is the constant (C) region. This is constant within a class of antibodies, but it varies between classes. The constant region will determine antibody's mechanism of action—for example whether it can bind complement proteins or stimulate mast cells and basophils to release histamine.
The five classes of antibodies are named IgA, IgD, IgE, IgG, and IgM. IgG accounts for approximately 80% of all immuniglobins.