Primary Immunodeficiency Diseases (PIDs) are rare hereditary and congenital disorders of the immune system. Lack of awareness amongst physicians of these rare diseases results in long delay in their diagnosis and treatment. This delay can amount to approximately six years for primary antibody deficiency disease, and by the time patients are diagnosed, they are already suffering from complications such as bronchiectasis and chronic sinusitis. These complications can be avoided, in the majority of patients, by early diagnosis and adequate treatment with immunoglobulin replacement therapy. Bone Marrow Transplantation (BMT) now offers the chance for curative treatment for some of these diseases, but is limited by the shortage of suitable matching donors, and by complications that arise from engraftment of donor cells. For these reasons BMT is applicable to only a proportion of cases. Somatic gene therapy allows the transplantation of new genes into the patients' own bone marrow to directly complement the genetic mutation, and restore full function to the white blood cells.