[Parasitic evasion mechanisms: Leishmania as an example].

Leishmania display a variety of mechanisms for effective evasion of the humoral and cellular immune responses of the host which are strongly associated with the expression of two major surface glycoconjugates, gp63 and lipophosphoglycan. The parasites are poor activators of the alternative complement pathway thus avoiding their own extracellular lysis. Complement bound on the surface of promastigotes promotes the uptake of leishmania by macrophages which function as "safe targets" as long as they are not activated by T lymphocytes. This is due to the fact that intracellular parasites are able to 1. decrease the oxidative burst; 2. scavenge toxic oxygen metabolites; 3. inhibit degradative lysosomal enzymes; 4. exploit the acidic milieu of lysosomes for their own metabolism. Finally, leishmania have been shown to evade the host's cellular immune response by down-regulating T cell-activating processes and by initiating the expansion of T cell subpopulations which promote their own survival.