Forty years after Bruton's discovery, the spectrum of primary defects of immunoglobulins has been largely extended and characterized. An increasingly more accurate recognition of the basic pathogenetic mechanisms of disease has helped to design more effective drugs and therapeutic strategies for patients with both primary and secondary immune deficiencies. In recent years, major advances in molecular biology have allowed characterization of the genetic basis of many primary immunodeficiencies, resulting in more accurate genetic counseling and leading to the first successful application of genetic therapy to the treatment of a human disease.