Gamma Globulin, mixture of proteins in plasma, the fluid portion of blood. It contains antibodies produced by specialized white blood cells, called lymphocytes, in the blood, liver, spleen, bone marrow, and lymph glands to protect the body from invading microorganisms. Each invading microorganism, such as a virus or a bacterium, carries unique substances called antigens that identify it as an invader. Every antigen stimulates production of a specific antibody, which circulates in the blood for a period of time and attacks the microorganism. Because the gamma globulin contains these antibodies, it is sometimes taken from patients who have recovered from chicken pox, hepatitis, and other infectious diseases and given to confer a rapid but short-term immunity on persons recently exposed to those diseases. Persons who suffer from an unusual deficiency of gamma globulin known as agammaglobulinemia are deficient in antibodies and may require periodic infusions of gamma globulin to maintain protection.