Strategies of obligate intracellular parasites for evading host defences.

During the course of establishing infection in a susceptible host, obligate intracellular parasites evade host defence mechanisms before, during and after entry into host cells. Before entry they circumvent the lytic activity of the complement cascade, during cell entry they avoid being killed by toxic oxygen metabolites and after entry they escape nonoxidative killing mechanisms such as degradation by lysosomal hydrolases. Different intracellular parasites, exemplified here by Leishmania spp, Trypanosoma cruzi and Toxoplasma gondii, undermine host defences at each step by various strategies that ultimately ensure their targeting to, and survival in, an appropriate intracellular compartment.