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Immune serum agglutinated merozoites of Plasmodium knowlesi in culture. Agglutinated merozoites attached to erythorcytes but were usually unable to invade. Marked aggregation of merozoites was associated with reduced invasion of erythrocytes in these cultures. The agglutination and reduced invasion were immunologically specific, for the effect of serum was greatest against homologous strains of P. knowlesi. Merozoite agglutination was caused by the binding of surface coats on adjacent parasites. This coat appeared on the plasma membrane of merozoites after it was exposed to culture medium, both with and without immune serum. The coat consisted of protein or glycoprotein, since it was susceptible to trypsin treatment. It appears that antibodies directed against this surface coat are crucial for reduced invasion of erythrocytes, at least in vitro.
Miller et al. (Tue,) studied this question.