Abstract
In malaria infections it is a common observation that some people are more resistant than others, an observation confirmed experimentally by Celli in 1901 1 and more recently by Yorke and Macfie, 2 who showed that there are some individuals who are resistant to experimental infection.
An experimental study of this problem with malaria parasites was out of the question until the advent of the treatment of paresis by a superimposed malaria infection. We, therefore, decided to study the problem in the case of another protozoan infection, the trypanosome, which produced in experimental animals a relapsing, fatal, disease.
Our studies thus far have brought out a number of facts bearing on this problem:
(1) Acquired immunity after cure. None of the animals, rabbits or guinea pigs, used in our experiments, and the number is now fairly large, failed to become infected, or recovered spontaneously from the infection. However, animals cured of the infection with Bayer 205 acquire a resistance to reinfection quite distinct from the possible protective action due to the drug itself. These experiments have been repeated many times, with both rabbits and guinea pigs and the result is quite constant. Infected animals treated and cured with minimal amounts of Bayer 205 (0.05 gm. per kilo), which in control animals gave only temporary protection, 3 became refractory to repeated attempts at reinfection for at least three to five months. Kleine and Fisher, 4 working with monkeys, also found that the cured animals were more resistant to reinfection than normal animals receiving the same dose of the drug.
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