This article discusses the dramatic events that accompany thymic involution during pregnancy, and traces the possible immune, neural and endocrine interactions that may occur. Ann Clarke and Marion Kendall present accumulating evidence that activity, not inactivity, is a feature of the thymus at this time. Whilst the cortex shrinks, the medulla enlarges and rearranges to create a microen;, ironment containing increased numbers of mature thymocytes. It is suggested that these recently derived T cells may contribute to the unique populations of cells with suppressive function that appear during pregnano,, and thereby contribute to the immune suppression of the mother to paternal and fetal antigens. In addition, the pregnancy-associated cortical involution of the thymus may reflect the deletion of clones with potential reactivity to paternal and~ or fetal antigens.

The thymus generates a diverse repertoire of T cells that are restricted to self major histocompatibility complex (MHC) and are self-tolerant I. As Fig. 1 indicates, the thymus changes dramatically throughot~ t pregnancy, and this has raised several questions: does the T-cell repertoire alter in the maternal thymus during these changes? Is the thymus involved in the appearance of no~ 21 T cells in the uterus arid placenta? What part does exposure to paternal and fetal antigens play? Are neural or hormonal signals involved? How does maternal physiology influence or direct these events? Current knowledge of neuroendocrine and immune interactions with the thymus has begun to provide a basis for answerlng these questions.