Gamma globulin, also called immunoglobulin is a class of blood plasma proteins, most notably including the antibodies that help fight infections and disease. Abnormal amounts of gamma globulin can have adverse effects on health or can be indicative of disease. In medicine, gamma globulin injections are used to treat certain conditions.

Gamma globulin injections can help boost a patient's immune system temporarily, so they are sometimes administered after a patient has been exposed to a contagious illness, though this practice is less common than it once was. This type of gamma globulin injection was formerly common for measles and hepatitis A, but vaccines currently exist for both conditions. Gamma globulin injections may also be given to patients who do not produce enough antibodies on their own as the result of a genetic disorder or an acquired condition.

Gamma globulin may also be used to treat immunological diseases other than deficiencies. For example, in immunological thrombocytopenia purpura, the patient's antibodies attack his or her own platelets, interfering with the blood's ability to clot. Gamma globulin injections are useful in treating this condition, though the mechanism by which they work is not fully understood. The injections may cause the spleen to ignore signals to destroy the antibody-tagged platelets, or they may cause the malfunctioning gamma globulin to degrade at an increased rate. In any case, the extra gamma globulin counteracts the malfunctioning antibodies that attack platelets and allows the platelets to thrive.

Since antibodies are used to fight infection, an unusually high amount of gamma globulin in the body, a condition known as hypergammaglobulinemia, is often a sign of infection. A proliferation of abnormal gamma globulin, or paraproteins, is likewise a sign of immune malfunction. Diseases of the gamma globulin such as this are called gammopathy. Gammopathy in itself may not be harmful, but it may be the sign of a serious immune condition, such as AIDS, or progress to a dangerous condition, such as nerve damage or plasma cell cancer. Therefore, close monitoring is advised.