In comparison to a large spectrum of currently available injectable vaccines, the nonparenteral route of immunization is used infrequently in both human and veterinary medicine. This is quite paradoxical in view of the fact that the majority of infectious diseases is encountered through the large surface area of mucosal membranes and, therefore, it would seem highly desirable to induce protective antibody-and cell-mediated immunity at the portals of entry of infectious agents (1). Because this is not commonly achieved by the current parenteral immunization protocols, alternative approaches briefly reviewed here have been considered to attain this goal.

Surprisingly, most immunologists usually do not realize that lymphoid tissues associated with mucosal membranes contain more immunocytes, including B and T lymphocytes, and plasma cells than any other tissue in the body. For example, 1 m of human small intestine contains 101 immunocytes (2); when multiplied by the total length of the intestine, this organ alone is by far the richest lymphoid tissue in the organism. The primary function of the mucosaassociated immune system is to prevent the pene-