How protozoan parasites evade the immune response

Protozoan pathogens such as Plasmodium, Leishmania, Trypanosoma and Entamoeba are responsible for several of the most widespread and lethal human diseases. Their successful survival depends mainly on evading the host immune system by, for example, penetrating and multiplying within cells, varying their surface antigens, eliminating their protein coat, and modulating the host immune response. Immunosuppression is sometimes caused directly by parasite products and sometimes involves antigenic mimicry, which often appears in association with parasitic diseases. However, one of the most sophisticated mechanisms of evasion is the selective activation of a subset of T helper cells. The present review describes mechanisms used by some protozoa pathogenic to humans to evade the host immune response. Relatively complex mechanisms by which protozoa penetrate, multiply within the cells and survive are discussed.