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A minor Plasmodium knowlesi 66 kD antigen, which plays an essential role in merozoite invasion, has been shown to be stable in distinct variants and strains of the parasite, and in the face of a specific immune response from the host. Parasites were unable to produce novel molecule(s) to replace it functionally, even in the presence of specific immune pressure. Rhesus monkeys immunized with the purified 66 kD antigen, with saponin as adjuvant, produced antibody which inhibited merozoite invasion of red cells in vitro. Four out of six immunized rhesus monkeys demonstrated clinically effective immunity when challenged at a time of known or presumed high inhibitory antibody titre. When immunization failed to protect, it was ascribed to insufficient levels of specific antibody attributable either to a suboptimal dose of antigen or the use of an inadequate adjuvant.

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