Studies of infections with plasmodium ovale. V. The effects of multiple inoculations upon the degree and nature of the immunity developed

An analysis has been made of the effects of multiple inoculations of P. ovale upon the degree of immunity developed against reinoculation with the same parasite. These results have been compared with those recorded by other workers with infections of P. vivax, P. malariae, P. falciparum and P. knowlesi. An attempt has also been made to obtain some indication of the relative rate of development of the anti-toxic factor in the immunity acquired and of its duration as compared with the anti-parasitic one. Under the conditions of our investigations much evidence has been found to show that:— As in infections with other plasmodia in man, successive inoculations with the same species of Plasmodium give rise to a higher degree of resistance to subsequent reinoculation than does a single infection. The degree of resistance appears to increase with each successive inoculation. The anti-toxic element of the immunity appears to be developed more rapidly than the anti-parasitic one, and the former element seems to reach an effective degree more rapidly in ovale infections than in infections with the other three human malaria parasites. There is evidence to suggest that the duration of the efficacy of the anti-parasitic element persists for a longer time than does the anti-toxic one, in the absence of renewed antigenic stimulation by fresh parasites.