globulin, any of a large family of proteins of a spherical or globular shape that are widely distributed throughout the plant and animal kingdoms. Many of them have been prepared in pure crystalline form. The term globulin is a partly procedural one, used in classifying an otherwise diverse group of proteins that are soluble in water or dilute salt solutions. Among the most important are the immunoglobulins (Ig), the antibodies of the immune system (see immunity). They are classified into five types based upon structure: IgA, IgD, IgE, IgG, and IgM. IgG or γ-globulin is the most common and forms about 70% of the immunoglobins in the blood. Other globulins are involved in the transport of a variety of substances, including lipids, hormones, and inorganic ions.

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.

Any of a major class of proteins insoluble in pure water and soluble in dilute saline (salt) solutions. In their natural state, the protein chain is folded into a globular form. Globulins are found in many plants, especially cereals. Globulins in animal fluids include enzymes, antibodies (the gamma globulins), lipoproteins, complement components, transport proteins, and various types of fibrous and contractile proteins.

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Subgroup of the globulins. In humans and many other mammals, most antibodies are in the gamma globulin fraction of blood. A human gamma globulin preparation may be administered (by injection) to persons lacking immunity, either generally or to a particular disease, after exposure or before expected exposure.

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