gamma globulin, a group of globulin proteins in human blood plasma, including most antibodies. These antibody substances are produced as a protective reaction of the body's immune system to the invasion of disease-producing organisms (see immunity). Injections of gamma globulin are used to create a rapid but temporary immunity in patients who have been exposed to certain diseases. Children who have been exposed to, but are not immunized against, measles and patients with hepatitis receive some protection from gamma globulin when it is administered during the incubation period of the infection. The gamma globulin used for such purposes is extracted from blood plasma from a large, diverse adult population; the resulting mixture is thus likely to contain antibodies from individuals who had been exposed to the appropriate infections.