In view of the circumstance that recovery from an induced malaria infection is attributable to the acquirement of a very potent immunity to the strain of parasite which induced the attack, it appeared desirable to ascertain whether the level of the immunity might be still further raised by a series of reinoculations with large doses of the same strain of living parasites. Early efforts in this direction demonstrated the undesirability of initiating the reinoculations while the primary parasitemia remained at microscopical levels, and the need for spacing the reinoculations sufficiently far apart so that they would be separated by an interval greater than the likely prepatent period. When these conditions are observed the early reinoculations may be followed, after the lapse of a prepatent period, by the return of a subclinical parasitemia which is probably a superinfection. A point is soon reached when the only parasites encountered will be those noted on the day of, and following reinoculations, which represent the inoculum, and that later inoculations will not be followed by any microscopically demonstrable parasitemia. When this point is reached the individual receiving the reinoculations may be regarded as hyperimmune.
The fully hyperimmune individual is able to withstand doses of parasites many million times greater than the minimal infecting dose without exhibiting either any clinical reaction or a subclinical parasitemia.
If this potent immunity is ascribable in any degree to circulating antibodies it should be capable of passive transfer from a hyperimmune to a susceptible person, through the transfusion of a large volume of blood. We have not been able to demonstrate that the blood of hyperimmune patients, transfused in 500 cc. quantities, has either prevented an infection, or modified the course of a developed infection, in susceptible persons, or in the latter case exerted any more effect than a similar quantity of blood from a person without previous malaria experience.