Fetal exposure to intact immunoglobulin E occurs via the gastrointestinal tract.

BACKGROUND: Consideration of the evolutionary significance of IgE might provide insight into the immunological interactions occurring in utero and during early post-natal life that regulate later atopic disease. OBJECTIVE: We postulated that the fetal gut is exposed to intact amniotic fluid IgE that might interact with local IgE receptors. METHODS: IgE levels in matched maternal blood and amniotic fluid (n = 47) or breast milk (n = 15) collected from pregnant women in the UK (Southampton) and Brazil (Sao Paulo) were studied. Expression of IgE receptors, Fc epsilon RI and Fc epsilon RII (CD23), in fetal gastrointestinal tract (n = 19) and skin (n = 11) was examined immunohistochemically. RESULTS: Human amniotic fluid at 16-18 weeks' gestation contained intact IgE at levels that increased as maternal circulating levels increased (Spearman's rho = 0.897; P < 0.001). Circulating IgE levels from women in Sao Paulo, Brazil, associated positively not only with term (> 37 weeks' gestation) amniotic fluid (rho = 0.993; P < 0.001) but also breast milk IgE levels (rho = 0.785; P = 0.001). Maternal levels of IgE did not change significantly over pregnancy and fetal circulating levels of IgE were very low (< 0.6 IU/mL). Low-affinity IgE receptors (CD23) were expressed in lymphoid follicles of the fetal gut from 16 weeks of gestation (6/8), but not from 11 to 16 weeks (0/11) or in the skin. CONCLUSION: Amniotic fluid contains intact IgE that might bind to CD23+ cells within the lymphoid follicles of the fetal gastrointestinal tract. The evolutionary significance of these interactions might be to prepare the immune system for helminthic parasite exposure at birth via IgE-mediated antigen focusing, or "education" of the developing immune system about the prevailing extrauterine environment. However, at present in societies where helminthosis is not a significant health issue, this pathway may still be operational and associated with the development of atopic disease.