1 Depts of Immunology and Pediatrics, Hospital 12 de Octubre, 28041 Madrid, Spain
Severe combined immunodeficiency is a heterogeneous group of disorders occurring mostly in infants with severe and potentially fatal defects of both cell-mediated and humoral immunity. Failure to thrive, intractable diarrhea, and pneumonia are their most prominent clinical features. However, although great progress has been made, only one of these syndromes is presently understood at the molecular level, namely, the adenosine deaminase deficiency, for which gene therapy is therefore under investigation. severe combined immunodeficiency in all other patients (about 80%) is mainly classified according to the absolute circulating T and B cell numbers, which show considerable variation: absent T and B cells with marked leukopenia (reticular dysgenesis); very low or absent T and B cells (Swiss-type severe combined immunodeficiency); low T and normal B cell numbers. In addition to these formal subgroups, several variants have been described that add to the classification difficulties in this area (ie, cell-mediated immunity defects with normal circulating B cells and immunoglobulin levels or Nezelof syndrome; severe combined immunodeficiency with normal T and B cell numbers).