Guinea-pigs and rats possess natural immunity to a high degree against the Leishmania of the Mediterranean basin in the flagellate stage in cultures on blood-agar (method of Novy-McNeal-Nicolle).
This immunity is exclusively of phagocytic nature. The Protozoa injected into the peritoneum become rapidly engulfed by the leucocytes, the mononuclears exclusively, and undergo gradual and progressive alterations, ending in their complete destruction, so that by the end of an hour or an hour and a half after the injection of 2 c.c. of a culture in full development, free flagellates are no longer to be found in the serum, and after two or three hours even the last vestiges of them are almost destroyed in the protoplasm of the phagocytes.
The Protozoa do not pass beyond the barrier of the peritoneum and do not find their way in this manner into the blood or the internal organs. These data of observation receive full confirmation in the results of the cultures. It is possible, in fact, to obtain subcultures from the peritoneal fluid up to about one hour after the injection of 2 c.c. of liquid of condensation very rich in Leishmanias, but not beyond this period, nor are subcultures obtained from the blood or organs of the animals in experiment.