Immunity to schistosomes in man and snail
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Schistosomiasis is a parasitic disease found in tropical areas, caused by fluke worms of the genus Schistosoma (S.). This parasite can infect humans and other mammals, and freshwater snails are the intermediate host. In humans infections can cause acute schistosomiasis, or chronic infections where the schistosomes can stay in the body for several years. These infections might result in fibrosis in different organs, including the urinary tract, intestines, liver and spleen, with a possible fatal outcome. Infections in snails result in an increased mortality, and a lower reproduction thus making the infections in snails more severe. In this report the differences and similarities between snail and human immune responses against schistosomiasis are described. The most important similarities are encapsulation of the parasite, the mechanism of killing and the similarity between human antibodies and snail proteins called FREPs, both play a role in neutralizing and precipitating proteins from schistosomes. Even though snails lack an adaptive response, the rate of resistance is higher in snails compared to humans. Some theories to explain this are proposed, including a faster response neutralisation in snails, a higher evolution rate and a higher selection pressure.