Immunology of the human genital tract

Despite much interest in the mechanisms of immune protection against sexually transmitted diseases (STDs), little is known about the role of the immune system in the genital tract. A better knowledge is needed to understand not only host protection against STDs, but also how tolerance is established in pregnancy to avoid rejection of the foetus.

The immune system of the genital tract displays characteristic features that are unique, and therefore distinct from those of other mucosal and systemic immune sites. It is functionally separate from the mucosal immune systems of the lung or intestine, and contrary to these systems, antibodies in the genital tract are dominated by IgG and not IgA. Most of the IgA is polymeric and consists of equal proportions IgA1 and IgA2. Polymeric IgA is actively transported via the polymeric immunoglobulin receptor on the basolateral surface of the epithelial cell, whereas it is not known how IgG antibodies are secreted. Antibody levels and isotypes exhibit strong hormonal dependence. Less is known about cell-mediated immune responses in the genital tract. Interest has focused on adhesion molecules, the existence of regulatory T and natural killer cells, and whether innate and early adaptive immune responses may be stimulated by local vaginal, intranasal or intestinal vaccinations. These topics are reviewed here and the most recent developments in these areas are reported.

A greater knowledge of immune activation and the homing of leukocytes to the genital tract is important for future attempts to design vaccines against STDs, as well as in understanding foetal tolerance.