Prenatal exposure to house dust mite allergen (Der p 1), cord blood T cell phenotype and cytokine production and atopic dermatitis during the first year of life House dust mites (HDM) are one of the major sources of indoor allergens in temperate humid areas. Exposure to HDM is an important risk factor for sensitization and subsequent development of allergic disorders in genetic susceptible patients (1–3). The process of sensi- tization is influenced by many factors, such as the dose and time of allergen exposure. For HDM allergens, there is a relationship between the dose of mite allergen exposure and development of Hagendorens MM, Ebo DG, Bridts CH, Van de Water L, De Clerck LS, Stevens WJ. Prenatal exposure to house dust mite allergen (Der p 1), cord blood T cell phenotype and cytokine production and atopic dermatitis during the first year of life. Pediatr Allergy Immunol 2004: 15: 308–315. Ó 2004 Blackwell Munksgaard This study investigated the influence of prenatal exposure to house dust mite (HDM, D. pteronyssinus) on interleukin (IL)-2, interferon-gamma (IFN-c) and IL-4 producing CD4+ and CD8+ T lymphocytes in cord blood as well as on the development of sensitization and occurrence of atopic dermatitis (AD) as the first symptom of allergy during the first year of life. Dust samples (n ¼ 22) were collected by vacuum cleaning the maternal mattress during early to mid-pregnancy. In these samples, the amount of the major HDM antigen (Der p 1) was assessed by a sensitive enzyme-linked immunosorbent assay technique (detection limit 0.004 lg/g dust). Flow cytometry was used to determine cord blood lymphocyte subtypes and to quantify the intracellular amounts of IL-2, IFN-c and IL-4 produced by cord blood CD4+ helper and CD8+ cytotoxic T lymphocytes, both spontaneously and after stimulation with phorbol-12-mirystate-13-acetate and ionomycin. Children were fol- lowed for 1 yr for the presence of symptoms associated with allergy. In addition, at the age of 1 yr specific IgE to different classical inhalant and food allergens was measured. Higher prenatal exposure to Der p 1 (>0.2 lg/g dust) was associated with a significant lower percentage of IFN-c producing stimulated CD4+ T lymphocytes, compared with lower prenatal Der p 1 exposure (p ¼ 0.03). The presence of AD during the first year of life (n ¼ 9) was associated with an increased number of naive CD4+ CD45RA+ lymphocytes (p ¼ 0.03), with an increased spontaneous IL-4 production by CD8+ lymphocytes (p ¼ 0.04) and with a decreased percentage of IFN-c producing stimulated CD4+ lymphocytes (p ¼ 0.04). Furthermore, exposure to HDM during preg- nancy tended to be higher in mothers of children with AD during the first year of life when compared with those without AD (p ¼ 0.08). This study shows that the level of prenatal exposure to Der p 1 influences the immune profile of cord blood T lymphocytes and the clinical outcome in early life. Therefore, the prenatal environment must be regarded as a possible early risk factors for allergic diseases in children. Margo M. Hagendorens 1,2 , Didier G. Ebo 1 , Chris H. Bridts 1 , Leen Van de Water 1 , Luc S. De Clerck 1 and Wim J. Stevens 1 Departments of 1 Immunology and 2 Paediatrics, University of Antwerp, Antwerp, Belgium Key words: house dust mite; cord blood; cytokines; atopic dermatitis; Der p 1; T lymphocyte Professor W.J. Stevens, Department of Immunology, University of Antwerp, Universiteitsplein 1 - 2610 Antwerp, Belgium Tel.: ++32 3 820 25 95 Fax: ++32 3 820 26 55 E-mail: Wim.Stevens@ua.ac.be Accepted 28 June 2004 Pediatr Allergy Immunol 2004: 15: 308–315 Printed in UK. All rights reserved Copyright Ó 2004 Blackwell Munksgaard PEDIATRIC ALLERGY AND IMMUNOLOGY 308