Parasites Immunity To

Frank EG Cox, Immunology Section, Division of Life Sciences, King's College London, London, UK

The term parasitism describes any relationship between two organisms in which one lives at the expense of the other and thus includes infections caused by prions, viruses, bacteria, fungi, protozoa and helminth worms. The word parasite, however, is traditionally reserved for the eukaryotes, protozoa and helminths. Protozoa are small single-celled microparasites, mostly measuring 1-10 p.m, which multiply within their definitive hosts in the same way as viruses and bacteria. Helminth worms, on the other hand, are macroparasites; they are much larger multicellular organisms, measuring 100 pm to 1000 |xm or more, and do not multiply within their definitive hosts. Parasites are found in every kind of invertebrate and vertebrate and individual hosts often harbor several different species, sometimes in massive numbers.

Humans, partly because of our lifestyles and habits, seem to be particularly susceptible to infections with parasites and over 50 species of protozoa belonging to 20 genera, and over 340 species of helminths belonging to about 50 genera have been recorded. These parasites of humans have attracted considerable attention because they cause some of the most widespread human diseases such as malaria, which affects 350-500 million people, schistosomiasis, which affects 200 million people, and as-cariasis, which affects some 250 million people-worldwide. In Africa, the presence of the parasitic protozoan responsible for trypanosomiasis prevents the use of over 10 million km2 of land in the sub-Saharan region for cattle rearing. Parasites of domestic animals are also of increasing importance; coc-cidiosis, for example, is a major threat to battery fowl, particularly in industrialized countries.

Parasitic diseases are difficult to control because they often produce intractable conditions, largely as a result of the inability of the immune system to keep them in check.

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