Gamma globulin, portion of blood protein containing antibodies. Several types of gamma globulin are recognized. Although they share basic structural features, they differ in size, site, behavior, and response to different antigens. Absence of all or some gamma globulins causes disorders of immunity, increasing susceptibility to infection, while the excessive formation of one type is the basis for myeloma, a disease characterized by bone pain, pathological fractures, and liability to infection. Gamma globulin is available for replacement therapy, and a type from highly immune subjects is sometimes used to protect against certain diseases (e.g., serum hepatitis, tetanus).