Splenectomized chicks were severely deficient in their ability to combat an infection or reinfection with Plasmodium lophurae. Infection of splenectomized chicks usually caused death while sham-operated controls recovered. Birds splenectomized on the 5th day of an infection, when parasitemias had dropped to a low level, showed an immediate and usually fatal recrudescence. Chicks which were hormonally or surgically bursectomized on the 1st day after hatching demonstrated higher parasitemias then their sham-operated controls. However, parasitemias of birds bursectomized at 18 days of age did not differ from their control group. Thymectomy had no significant effect on the resultant parasitemias. It is thus concluded that the spleen is of primary importance in the development of immunity to this strain of Plasmodium lophurae. The bursa is of secondary importance and contributes something to the development of the immune mechanism before 18 days of age.

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