The immunoglobulin system initially evolved as a defense system to maintain genetic stability during evolution. Studies in the past two decades have resulted in the elucidation of immunoglobulin structure and the definition of their chemical composition. Disorders of immunoglobulin production have led to an enhanced understanding of the functional aspects of immunoglobulin chemistry. The recognition of these disorders has enhanced our knowledge of antibody specificity and led to the development of immunochemical techniques such as radioimmunoassay as well as the development of techniques of transplantation and immune reconstitution in man.