Diagnosis and classification of severe combined immunodeficiency disease.

The failure to demonstrate normal humoral and cell-mediated immunity (CMI) in patients diagnosed as SCID is seen to reflect the varied pathogenesis of this syndrome. Two major groups of patients have been described, those with or without an associated absence of the enzyme ADA. The heterogeneity of the syndrome is expressed in variable inheritance patterns (particularly defined X-linked or autosomal recessive modes of inheritance), differing clinical presentations, and significant variability in laboratory findings. Some of this heterogeneity of laboratory findings may in fact be contributed to by the high incidence of infection or engraftment of maternal cells in utero. Common to all, however, is the profound deficiency of functional attributes of humoral and cell-mediated immunity. Insight into the biology of this immunodeficiency has advanced steadily in the last decade. Although initially hypothesized to represent a primary lymphoid stem-cell defect, newer technologies to identify and enumerate lymphocyte subpopulations and precursor lymphocytes have revealed the complexity of the disorder. This complexity may now be attributable to a number of abnormalities in the quantitative and qualitative differentiation of these lymphoid stem cells. Functional differentiation of lymphocytes is the result of a progressive and orderly sequence of events. In SCID, lymphocytes of both lineages may be arrested at specific and identifiable stages of maturation, leading to a deficiency of cell-mediated and humoral immunity. In many patients with SCID, the combined immune deficiency may be linked solely to a failure in the stepwise progression of T-cell differentiation.