Immunity in malaria

Abstract

1. 1. Epidemiological observations on the acquired immunity of natives resident where malaria is hyperendemic indicate that the population is largely protected during adult life from the more acute manifestations of the disease. This protection is acquired gradually, is dependent on practically continuous infection and exposure to the bites of anopheline mosquitoes over a period of twelve years or longer, and results in a state which has been referred to as “tolerance,” “ premunition,” “labile immunity” and “relative immunity.” Coincidently with the development of this partial, relative or tolerant immunity, the spleen rate and the parasite rate show a steady decline while the organisms, so far as numbers are concerned, diminish almost to vanishing point in the blood of infected persons. The acquisition of such a relative or tolerant immunity by Europeans is fraught with many dangers chief among which may be reckoned blackwater fever, a sequel to malaria seen only with the utmost rarity, if at all, during the “tolerant immunisation” of the native residents. This last fact would seem to suggest that some form or other of hereditary or racial immunity may be engendered by the continued malarial saturation through many generations of a nation or tribe. 2. 2. The experimental work on induced malaria has now proved that persons, who are being subjected to infection for the first time, vary as regards their susceptibility within very wide limits from a state of complete refractoriness, through the different grades of resistance, to a degree of great susceptibility. After spontaneous recovery from benign tertian malaria there develops an immunity to superinfection, which, however, is only directed against the same strain of parasite since re-infection can be produced by inoculation with either another species of parasite or even a different strain of the original species. From this it may be deduced that the protective mechanism, possibly antibody formation, is probably of a specific character. Proof is still lacking as to whether the resistance of persons with acquired immunity is due to the presence of latent infections or is in the nature of a true immunity persisting even after the complete disappearance of all parasites from the body. 3. 3. While attempts to demonstrate the presence of protective antibodies have so far been unconvincing, the evidence derived from serological and other studies tends to support the view that such antibodies do exist. 4. 4. Canaries, which have recovered from the acute initial attack of malaria, may harbour parasites in their bodies for years ( mazza , 1924 ), and during this period of latency they are immune to superinfection. This acquired immunity is to a certain extent specific. Antibodies have not been demonstrated conclusively in immune birds but there is a definite increase in the activity of the reticulo-endothelial system of cells especially of the liver and spleen. Phagocytosis would appear to be a factor of importance in this state of latency or tolerance. 5. 5. Phagocytosis undoubtedly plays an important part as a controlling factor in malarial infections of man. It may be observed in the peripheral blood and also in smears of the internal organs and is not merely confined to the ingestion of free pigment and dead parasites. Although the macrophagic activity probably reaches its maximum at the time when the schizonts are mature it nevertheless may persist throughout the whole cycle. The tendency for red blood corpuscles containing forms of P. falciparum to adhere to the lining endothelium of the capillaries, and also, in culture, to aggregate together and clump around the reticulo-endothelial elements is suggestive of the production of some antibody.