1. 1. It has been shown that the older the mosquito Aedes aegypti grows the more resistant it becomes to infection with the malarial parasite Plasmodium gallinaceum . Thus, the levels of infection, as measured by the parasite population of the host, in groups of aging mosquitoes, 2 to 4 weeks old, were progressively and significantly lower than the levels of infection in young, newly emerged, control groups.2. 2. It has been shown, too, that aging mosquitoes given a normal blood meal at least 9 days prior to the infective feed become again as susceptible to infection as newly emerged, young mosquitoes. Similarly, mosquitoes maintained on raisin infusions for a period of 4 weeks became again as susceptible to infection as young mosquitoes. On the other hand, aging mosquitoes fed on solutions of chick or human plasma, hemoglobin, lysed red cells, or plasma and lysed red cells combined, developed an even greater degree of resistance to infection than that which results as a consequence of advanced age.3. 3. These facts, and the fact that changes in tissue nitrogen levels of aging mosquitoes bore no demonstrable relation to changes in the innate immunity of such mosquitoes, suggest that host immunity to infection depends upon the addition or depletion of specific physiological or metabolic factors which function in low concentration, rather than upon gross quantitative changes in the nitrogen levels of the host animal.4. 4. Finally, from the observed differences in effect on host immunity between whole blood and separated blood elements, it appears not unreasonable to assume that there is present some labile component in whole blood, most probably associated with intact cells, which undergoes considerable qualitative change when cells are lysed or the blood elements are mechanically separated.
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