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Discuss the host-parasite relationship in terms of coexistence vs. conflict In order to understand the nature of the relationship between host and parasite, and the immunological response involved, it is necessary to distinguish between microparasites and macroparasites. Protozoa, like viruses and bacteria, are microparasites that multiply within their vertebrate hosts posing an immediate threat unless constrained. Helminths, on the other hand, are macroparasites that do not normally multiply within their vertebrate hosts and only present a real threat when large numbers have been built up following repeated exposure to infection. The host defence strategies must therefore be different in response to protozoa and helminths, and this is mediated by controlling the balance between two pathways (TH1 and TH2 pathways) which activate various cells in order to kill the parasite. It is important to note, however, that parasites have evolved various methods to evade the immune system, and this can not only result...

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