Background: Increasing evidence indicates a prominent role of allergen-specific TH2 cells, with high IL-4 and IL-5 production and low interferon-γ production, in the regulation of IgE and eosinophil production in allergic disorders. However, most studies have concentrated on T cells reactive with inhalation allergens, whereas little is known about the properties of food allergen-reactive T cells. Objective: In this study we therefore characterized peanut-specific T cells, cloned from a patient with severe peanut allergy. Methods: Peripheral blood mononudear cells from patients with peanut allergy and nonallergic individuals were stimulated with crude peanut extract (CPE) to compare the proliferative responses and to select a suitable patient for the cloning of CPE-specific T cells. The resultant panel of CPE-reactive T-lymphocyte clones was serologically phenotyped by flow cytometty and analyzed for cytokine secretion by ELISA. Results: The patients' peripheral blood mononudear cells showed a dose-dependent proliferative response to CPE, which was significantly higher (p < 0.05) than in peripheral blood mononudear cells of nonallergic donors. The CPE-specific T-lymphocyte clones generated from the selected patient were all CD4+/CD8- T helper cells with a TH2 cytokine profile, secreting high amounts of IL-4 and IL-5, but little or no interferon-γ. Conclusions: This study demonstrates that peanut-specific T cells do occur in the peripheral blood of patients with peanut allergy and suggests an increased frequency of these T cells in patients compared with nonallergic control subjects. The CD4+ phenotype and the TH2 cytokine profile of the CPE-specific T-lymphocyte clones suggest a functional role of allergen-specific TH2 cells in the pathophysiology of food allergy, similar to the function of inhalation allergen-specific TH2 cells. Copyright © 1996 by Mosby-Year Book, Inc.