Abstract
Allergen-specific reactivity at birth is now a commonly described phenomenon, and interest in this observation is heightened by the association between altered immunologic reactivity at this time and subsequent development of atopy. The lymphoid tissue or tissues in which antigen priming occurs during fetal development are not known, but in this issue of the Journal, Jones et al (p 235 ) provide evidence that the fetal gut is a plausible site at which antigen priming can occur. MHC class II+ cells, CD3+ T cells, and cells expressing the costimulatory molecules required to initiate an antigen-specific immune response are all present from 16 to 19 weeks of gestation, as are dietary antigen and cytokines (in amniotic fluid) that favor a TH2-biased response.
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