noun, plural: immunoglobulins
(1) Any of several classes of structurally related proteins in the blood serum, produced by plasma cells as an immune response against foreign agents (antigens).
(2) The fraction of the blood serum containing antibodies.
In placental mammals, there are about five classes of immunoglobulins based on structure and biological activity: IgA, IgD, IgE, IgG, IgM.
Word origin: immuno- (relating to immune) + globulin.