In this. address, the author brings together the important observations made by him and his co-workers on immunity to P. vivax infections during a prolonged experience of malaria therapy. A clinical attack of three weeks or longer after inoculation with P. vivax indicates that the patient was previously completely susceptible. Short attacks from one day to two weeks may follow either heterologous or homologous reinoculation, indicating, in the latter case, that immunity is incomplete. Homologous reinoculation may be followed by no clinical attack or fail altogether; the latter indicates a very effective refractory condition. A comparison of the parasite density at the beginning and the end of the clinical attack shows that a tolerance to the presence of the parasites is the first manifestation of developing immunity. Soon after the infection becomes latent, the host is able to destroy numbers of parasites enormously greater than the few which can initiate an infection in a susceptible person. This destructive mechanism does not seem to be operative against spprozoites, but the resulting trophozoites will quickly disappear. The state of complete refractoriness may be of considerable duration; there is no evidence that its persistence is dependent upon the persistence of latent infection. The failure of serum of hyperimmune persons to confer passive immunity on susceptible persons indicates that the immunity is not humoral.

The possibility of conferring active immunity by inoculation with killed parasites is difficult to determine. Two susceptible patients, saturated with quinacrine, were each given three inoculations of living parasite. 3, which were presumably killed in vivo. Subsequent reinoculation by infected mosquitoes was followed by only a short clinical attack in each case. Norman While.