Properties of protective malarial antibody.

Properties of protective malarial antibody have been studied in cultures of P. knowlesi giving average parasite multiplication rates of sixfold in 24 hours. Parasite growth was assessed by incorporation of [3H]-leucine into protein. Immune serum has little effect upon the growth of intracellular parasites, but prevents reinvasion of red cells and inhibits the succeeding cycle of parasite development. Protective antibody is present in relatively low titre in immune sera even after long immunization and this may explain certain characteristic features of malarial immunity. Protective antibody in the sera studied is associated with IgG and IgM; its action is not complement dependent, but requires at least two combining sites per molecule. Anti-malarial antibody has several features in common with viral neutralizing antibody.