Objective: To determine the relative frequencies of the different genetic forms of severe combined immunodeficiency (SCID) and whether there are distinctive characteristics of the particular genotypes.

Study design: The demographic, genetic, and immunologic features of 108 infants with SCID who were treated consecutively at Duke University Medical Center were analyzed.

Results: Eighty-nine subjects were boys and 19 were girls; there were 84 white infants, 16 black infants, and 8 Hispanic infants. Forty-nine had X-linked SCID with mutations of common cytokine receptor gamma chain (γ_c ), 16 had adenosine deaminase (ADA) deficiency, 8 had Janus kinase 3 (Jak3) deficiency, 21 had unknown autosomal recessive mutations, 1 had reticular dysgenesis, 1 had cartilage hair hypoplasia, and 12 (all boys) had SCID of undetermined type. Deficiency of ADA caused the most profound lymphopenia; γ_c or Jak3 deficiency resulted in the most B cells and fewest natural killer (NK) cells; NK cells and function were highest in autosomal recessive and unknown types of SCID.

Conclusions: Different SCID genotypes are associated with distinctive lymphocyte characteristics. The presence of NK function in ADA-deficient, autosomal recessive, and unknown type SCIDs, and low NK function in a majority of γ_c and Jak3 SCIDs indicates that some molecular lesions affect T, B, and NK cells (γ_c and Jak3), others primarily T cells (ADA deficiency), and others just T and B cells. (J Pediatr 1997;130:378-87)