• Definition: A rare genetic condition caused by numerous genetic mutations that result in the defective development of functional B cells and T cells.
• Etiology: various mutations, the most common of which are:
  • X-linked recessive: mutations in the gene encoding the common gamma chain → defective IL-2R gamma chain receptor linked to JAK3 (most common SCID mutation)
  • Autosomal recessive
    • Adenosine deaminase deficiency → accumulation of toxic metabolites (deoxyadenosine and dATP) and disrupted purine metabolism → accumulation of dATP inhibits the function of ribonucleotide reductase → impaired generation of deoxynucleotides