Mechanisms of defense against P. falciparum asexual blood stages in humans.

Based on the observation that host-parasite immune relationships depend not only on the parasite species but also on the host, we consider that P. falciparum can be best studied in humans. In vivo observations suggest that there are in humans a series of immunities towards P. falciparum: besides innate non-antigen-specific defenses, which play a major role, additional defense mechanisms are acquired after consecutive malaria attacks. They contribute to realize such various states of immunity as acquired resistance to cerebral malaria, anti-disease immunity, strain-specific immunity, and, after a long time of exposure in hyper- or holoendemic areas, the host-parasite equilibrium characteristic of premunition. Passive transfer experiments have established that IgG play a major role in premunition. We review, on the one hand, in vitro evidences that protective antibodies have the characteristic capacity of promoting a monocyte-dependent inhibition of parasite growth (ADCI) and, on the other hand, the in vivo observations that suggest that this mechanism of defense could be one of the immune foundations of the state of premunition.