Abstract
Athymic (nude) mice, their normal littermates, and Swiss white mice were infected with 750 oocysts of Eimeria falciformis var. pragensis and reinfected twice with 20,000 oocysts, 20 and 40 days after the primary infection. The prepatent and patent periods of the primary infection were similar in each group of mice; however, the athymic mice discharged more oocysts. The normal littermates and Swiss white mice developed immunity to the parasite after the first or second infection, whereas the athymic mice never developed immunity. Infection with E. falciformis var. pragensis induced the production of antibodies in the normal littermates and Swiss white mice, whereas antibody production was impaired in athymic mice. Passive administration of immune serum did not protect athymic mice from reinfection by the parasite. Immunity to the parasite was thymus dependent, and "effector" T-lymphocytes seemed to be required for protection.