AbstractWe characterized the defects of CD4+ cells in a 17‐month‐old girl suffering from combined immunodeficiency with hypereosinophilia (Omenn's syndrome). Because the vast majority of peripheral blood CD4+cells expressed the CD45R0 isoform, we purified circulating CD4+CD45R0+cells from the patient and healthy individuals in order to compare their production of cytokines. The patient's CD4+CD45R0+cells spontaneously produced high levels of interleukin‐5 (IL‐5)in vitro(1600 pg/ml after 24 h of culture) and this was associated with the presence of IL‐5 in serum (323 pg/ml). After stimulation with phorbol 12‐myristate 13‐acetate (PMA) and calcium ionophore A23187, they produced higher levels of IL‐4 (306vs.55 ± 4 pg/ml) and IL‐5 (2900vs.213 ± 72 pg/ml) and lower levels of IL‐2 (17 vs. 63 ± 17 IU/ml) and interferon‐γ (IFN‐γ) (16vs.299 ± 70 IU/ml) than controls CD4+CD45R0+cells. This T helper type 2 (Th2) pattern was confirmed by the detection using reverse polymerase chain reaction of IL‐4, IL‐5 and IL‐10 mRNA within peripheral blood mononuclear cells. During a therapeutic trial with human IFN‐γ (40 μg/day) which ameliorated the clinical status of the patient, we observed a down‐regulation of thein vivoexpression of IL‐5 and IL‐10, a normalization of the eosinophil count and an improvement of the Tcell response to phytohemagglutinin. This observation indicates for the first time that Th2‐like cells might be involved in certain forms of congenital immunodeficiency and that IF