The mucosal immune system of the female genital tract is under a strong hormonal control that regulates the transport of immunoglobulins, the levels of cytokines, the distribution of various cell populations, and the antigen presentation in the genital tissues during the reproductive cycle. Unique to the genital tract and unlike other mucosal secretions in which S-IgA is the dominant isotype, IgG levels in the lower genital tract secretions equal or exceed the levels of S-IgA. The presence of the mucosal effector sites and a significant contribution of the systemic immune compartment are of considerable functional importance in the stimulation of protective immune responses. Comparison of-the levels and molecular properties of Ig of the major isotypes in blood, external secretions such as saliva, milk, and intestinal fluid, and female genital tract secretions-indicate that in humans the systemic and mucosal compartments of the immune system display a considerable degree of independence. In contrast to the mucosal system of, for example, the alimentary tract-the mucosal system of the genital tract is characterized by a significant contribution of the systemic compartment with respect to-the Ig isotype distribution, unique distribution and phenotypes of B and T cells, strong hormonal dependency, lack of typical lymphoepithelial inductive sites, and relative weakness of immune responses to local infections or immunizations with certain antigens. © 2005 Elsevier Inc. All rights reserved.