Publisher Summary Techniques for the study of immunoglobulin metabolism using purified radioiodinated serum proteins helps define the factors controlling the rates of immunoglobulin synthesis, catabolism, and transport. These techniques are important in studying the pathogenesis of the abnormalities of immunoglobulin levels seen in various immunodeficiency diseases. These techniques are of value in the differential diagnosis of immunodeficiency states and in addition provide the information necessary for the rational use of γ-globulin or its subunits as therapeutic tools. The disorders of immunoglobulin pool size and concentration occur secondary to a variety of pathophysiological mechanisms that are associated with a variety of different patterns of immunoglobulin metabolism. At one end of the metabolic processes are the disorders characterized by decreased immunoglobulin synthesis, while on the other end of the metabolic process are disorders characterized by a short survival of immunoglobulin. This short immunoglobulin survival may be caused by excessive loss of immunoglobulins into the urinary or gastrointestinal tracts. Hypogammaglobulinemia may be caused by a disorder of endogenous catabolic pathways that affect a single immunoglobulin class or all immunoglobulin classes.