ABSTRACT
The term parasitism describes any relationship between two organisms in which one lives at the expense of the other and includes infections caused by prions, viruses, bacteria, fungi, protozoa, and helminth worms. Helminth worms are much larger multicellular organisms, measuring 100 um to 1000 urn or more. A number of organisms including viruses, bacteria, protozoa, and helminth worms inhabit the gastrointestinal tract. The immunology of malaria has been extensively studied, and it is known that there is a gradual build up of immunity over a period of many years. A major problem in malaria is that all the stages in the life cycle are antigenically distinct, and the acquisition of immunity involves a number of separate processes that must be controlled in terms of place, magnitude, and duration. Although parasites can survive in normal macrophages, they are killed by interferon-y activated macrophages, therefore the most important cells involved in immunity to toxoplasmosis are macrophages.
