Immunity to Protozoa
نویسندگان
چکیده
Parasites with complex life cycles often stimulate both antibodyand cell-mediated defence mechanisms, whose effectiveness depends on the particular parasite and the stage of infection. Malaria, which is caused by different species of plasmodia, ranks amongst the most common and important parasitic diseases. Infection of humans is initiatedby thebite of an infectedmosquito, resulting in the injection of sporozoite stages. The sporozoite enters hepatocytes where asexual reproduction (schizogony) takes place. Rupture of the infected cells releases thousandsofmerozoites,which thenpenetrate redbloodcells to initiate repeated erythrocytic cycles. Thus, the different stages of the parasite occur inside cells that either express (e.g. hepatocytes) or lack (e.g. erythrocytes) major histocompatibility complex (MHC) class I or class II molecules. Since T cells are able to recognize parasite antigens only as processed peptides presented by appropriateMHCmolecules, extracellular forms of the parasite, or parasite stages in erythrocytes devoid of the presentation machinery, are controlled mainly by antibodydependent acquired immune effector mechanisms. However, the immunological control is only partial: complete elimination of plasmodia is not achieved.
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