The antibody response to an epitope on gamete antigens of Plasmodium fakciparum in persons naturally exposed to malaria has been investigated by competitive enzyme-linked immunosorbent assay. The assay detects antibodies to an epitope on the 48/45-kilodalton (kDa) gamete surface antigen by competition with horseradish peroxidase-labeled monoclonal antibody 11C5-B10. Five sera previously shown to immunoprecipitate the 230and 48/45-kDa antigens significantly inhibited 1IC5-B10 binding to an average of 24.2% of control. The one serum which precipitated only the 48/45-kDa antigen did not inhibit IIC5-B10 binding. For 26 sera which were negative by immunoprecipitation, mean binding in the assay was 112.7% of control (pooled London nonimmune sera). Recognition of both 230-kDa and 48/45-kDa antigens was associated with a titer of 1:9 or greater (reciprocal geometric mean titer, 27.6) for inhibition to more than 2 standard deviations from the mean of the negative sera. The results show that the 1IC5-B10 binding site is a naturally immunogenic epitope recognized by the majority of persons who had antibodies to the 48/45-kDa protein. An additional finding was enhancement of binding of 1IC5-B10 to an average of 154.4% of control by five sera which recognized only the 230-kDa antigen, presumably due to conformational alteration of the gamete antigen complex.

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