Exam 13: Failures of the Bodys Defenses

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This strategy has been selected by evolution to maintain the long-term survival of the parasite in its host.The surface glycoproteins,known as variable surface glycoproteins (VSGs)are involved in enabling the parasite to evade the immune response of its host.The trypanosome can change the VSG that is expressed,thus rendering antibodies made against the previously expressed VSG ineffective at controlling the parasite.Switching occurs at random during the life cycle of the trypanosome by a process called gene conversion,in which a different VSG gene is rearranged to the single 'expression site,' replacing the original gene,and is expressed instead.This process is called antigenic variation and leads to the periodic rise and fall in the number of parasites that is characteristic of trypanosome infections.Antibodies made by the host against the VSG expressed on the infecting trypanosomes will start to suppress the infection and parasite numbers will fall.But if some parasites switch VSG,these antibodies will be ineffective,and parasites bearing the new VSG will multiply.Parasite numbers will rise for a time until the new immune response begins to suppress them in turn.By then a third VSG gene is likely to have been expressed,and parasites bearing that one will begin to predominate in the host,and so the cycle continues.