n. Abbr. Ig1. Any of a group of large glycoproteins, including all the antibodies that function in the vertebrate immune system, that have a structure consisting of two heavy chains and two light chains. There are five major classes of immunoglobulin isotypes: IgA, IgD, IgE, IgG, and IgM.
2. A preparation containing immunoglobulin proteins, especially immune globulin.