Deficiency of autologous mixed lymphocyte reactions and serum thymic factor level in Down's syndrome.

The immune competence of the T lymphocyte system was studied in 28 noninstitutionalized subjects with Down's syndrome (DS) and were compared with sex- and age-matched healthy controls. The ability of enriched T lymphocytes to respond to 3 different T cell stimulants revealed a selective impairment of T lymphocyte subset(s). Subjects with DS showed normal responsiveness in allogeneic mixed lymphocyte reactions, but their response to phytohemagglutinin and in autologous mixed lymphocyte reactions was severely impaired. Non-T cells from DS subjects stimulated equally well both normal and DS allogeneic T lymphocytes. The blood concentration of serum thymic factor in the majority of DS subjects was much lower than that found in age-matched healthy controls. These data support the hypothesis that a deficiency of the T-dependent regulatory system is an intrinsic feature of DS and confirm the precocious aging of the immune system in these subjects.