Dr. Kassai Tibor szerk.: Parasitologia Hungarica 1. (Budapest, 1968)
designed to study connections between parasitic infection acquired by juvenile hosts and their immune reactivity and concern modification in the host-parasite relationship. It was shown that, after an adequate infection of young rats by Nippostrongylus the characteristic immune-elimination of the primary population fails to appear and that the parasites survive even when the hosts are immunologically mature (Fig. 2). This status can best be attained by infection of rats younger than three weeks. The life-span and egg yield of parasite populations, arising from juvenile infections are longer than those of populations originating from infections .at a later age. According to the author, the comparative stability of early populations is attributable to production of antigens by such populations causing a specific decrease in the immune reactivity oX hosts when they become immunologically mature. Accordingly ,this type cf change in the host-parasite relationship should be regarded as a manifestation of tolerance. The early, primary population is expelled from most rats within 50-100 days,and the animals become more or less resistant to subsequent infection. Repeated infections apparently lengthen the life of the primary population but the host is able to inhibit the establishment of fresh infections (F i g. 5), Secondary infection of tolerant rats when immunologically nature in general resembles a primary reaction in normal rats. Expulsion, of the secondary population is, however, incomplete, permitting the survival of a stable residual worm population capable of prolonged egg production (F 1 g. 4). Reinfection of tolerant hosts about the time when they acquire immunological maturity induces resistance, to later homologous infection, but a parasite population capable of a low-grade, steady egg production will survive in such rats also (F ig. 5). The immunologically more or less tolerant state is, as a contributive factor, probably significant in the epidemiology ai heir minthoses involving strong immune reactions.
