Proteins that have the general structural features of antibodies but that have no known antigen-binding properties are known as immunoglobulins. Antibodies are symmetrical molecules made up of two identical glycosylated heavy chains (Mr 50-75 kDa) joined by disulfide bonds and two identical glycosylated light chains (Mr ~25 kDa) joined to the heavy chains by disulfide bonds. Immunoglobulin G (IgG) is one of the 5 main classes of immunoglobulins and is defined by the presence of a gamma heavy chain. There is only one subclass of IgG in the rabbit.