Severe combined immunodeficiences: new and old scenarios.

Severe combined immunodeficiencies (SCIDs) represent a group of distinct congenital disorders affecting either cell-mediated or humoral immunity, which lead invariably to severe and life-threatening infections. The different forms of SCID are currently classified according to the presence or absence of T, B, and NK cells. This greatly helps define the site of the blockage during the differentiation process.