Immunoglobulin (also called immune globulin or gamma globulin) is a protein in human blood and tissue fluids. These proteins are also called antibodies, which help the body's immune system recognize and destroy foreign substances, such as bacteria and viruses.

Immunoglobulin (IG) may be given to help prevent an illness after exposure to an infected person. It can also be given to people with certain immune system deficiencies to prevent infections. Immunoglobulin is usually taken from the blood of people recovering from the illness. For example, the immunoglobulin given to help prevent hepatitis A infection is taken from the blood of people who are recovering from hepatitis A virus infection.

The protection provided by an immunoglobulin injection lasts from days to months, depending on the disease.