Mechanisms of immunity to malaria.

The erythrocytic phase of malarial infection provides a potent stimulus for the production of specific malarial antibody. Serologic tests do not provide any indication of immune status, so that much specific antibody has no protective function. The role of serum antibody in acquired malarial immunity has, however, been established by passive transfer tests, and in the case of P. knowlesi by specific inhibition of the cyclic growth of parasites in vitro. The inhibitory antibody appears to combine with merozoites and prevents their attachment to red cells, thus interrupting the cyclic proliferation of the parasite. The inhibitory antibody response is predominantly variant-specific, but cross-reacting antibody occurs in sufficient amount to suppress proliferation of most other variants of the species. The occurrence of cross-immunity between variants is encouraging from the point of view of vaccination. If it were possible to isolate cross-reacting antigens, these could provide the basis for a malarial vaccine effective against erythrocytic forms of the parasite.