globulin, one of the major classifications of proteins, which may be further divided into the euglobulins and the pseudoglobulins. The former group is insoluble in water but soluble in saline solutions and may be precipitated in water that has been half-saturated with a salt such as ammonium sulfate. The latter group is soluble in water and has properties that resemble those of the true globulins. Globulins are an important source of protein in seed plants and are found in minute amounts in cereals. Globulins found in animal fluids are enzymes, antibodies, and fibrous and contractile proteins usually contained in the blood plasma.
Three types of globulin have been identified—alpha, beta, and gamma. Alpha and beta globulins are transport proteins, serve as substrates upon which other substances are formed, and perform other diverse functions. Gamma globulins have a vital role in natural and acquired immunity to infection. See also gamma globulin.