A variant of severe combined immunodeficiency with normal in vitro response to allogeneic cells and an increase in circulating B lymphocytes persisting several months after successful bone marrow graft.

Severe combined immunodeficiency was diagnosed soon after birth in an infant with a well documented familial history of recessive autosomal inheritance. No thymic shadow was detectable and the number of circulating lymphocytes was decreased. Both humoral and cell-mediated immunity were severely impaired. Despite the absence of response to stimulation by phytohaemagglutinin, the circulating lymphocytes responded normally to allogeneic cells in the mixed leucocyte culture reaction. In addition, the vast majority of the blood lymphocytes were B lymphocytes bearing membrane-bound immunoglobulins which were shown to be actual cell products. A foetal thymus graft was performed at the age of 1 month and was not followed by any improvement of the immunological parameters. The transplantation of unfractionated bone marrow cells from an HLA identical brother with negative reactions in mixed lymphocyte culture was performed 1 month later and resulted in a gradual restoration of immunological competence for both B-cell and T-cell functions. High numbers of circulating B lymphocytes persisted for several months after immunological reconstitution. These unusual findings argue against the hypothesis of a stem cell defect in the presently described variant and suggest that severe combined immunodeficiencies may represent a heterogeneous group of diseases with regard to the level of the basic defect.