Helminths at mucosal barriers--interaction with the immune system.

Helminth parasites are the cause of very significant morbidity, mortality and economic losses in man and domestic animals. Most parasitic helminths infect their hosts via the oral route, and live either at the mucosal surface of the gastro-intestinal tract (GIT), or cross this mucosal barrier on their way to predilection sites. Many helminths live at mucosal surfaces, typically the gut or respiratory tract, and some cross these barriers, either temporarily, spending a period of time in the mucosa before returning to the mucosal surface, or to access other tissues and sites in the host. Typically, helminths induce strongly polarised Th2 responses, which are often effective in mediating protective immunity against those parasites living at mucosal surfaces, but less so in protecting against tissue-dwelling parasites. Induction of strongly-polarised Th2 responses may impair the ability of parasites hosts to eliminate other pathogens. Control of helminth infections relies largely on chemotherapy, together with management and environmental measures designed to keep hosts away from infective stages. Drug resistance has become a significant problem in some helminth populations, and this has promoted interest in the development of immunoprophylactic strategies. However, despite intensive research efforts, helminth vaccines have not become part of regular control strategies. In addition to the considerable technical difficulties posed in the production of vaccines against these complex organisms, further difficulties in securing acceptance for anti-helminth vaccine by regulatory authorities and by users, will be encountered. Such vaccines need not result in sterile immunity, as is required of anti-bacterial and anti-viral vaccines. Recent evidence indicates that while helminths are responsible for disease, immunopathology and impairment of immunity to other pathogens, a complete absence of helminth infection during early life may be a predisposing factor for the development of auto-immune pathology.