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In the present experiment the response of sheep to a single infection with Haemonchus contortus at graded doses, and their responses to two subsequent infections with 3000 infective larvae, were studied. The response of the sheep was investigated by estimation of faecal worm egg counts on alternate days throughout the course of the experiment and by following the development of the parasite by differential worm counts performed on animals slaughtered at crucial times.Initial infections were established with graded doses of 500–2700 infective larvae. The proportion of infective larvae which became established at each dose, development of the worm burden and fecundity per adult worm were similar for each dose.Following first challenge infection with 3000 infective larvae there were no effects which could be related to the previous level of infection. Whereas there was no difference in egg production per female worm, a small but consistent increase in the number of retarded larvae was observed in the previously infected animals (6% of the burden was in the 4th larval stage at day 56) compared with the previously uninfected control animals (2% in the 4th stage at day 56).Following a second challenge with 3000 infective larvae an increase in the number of retarded larvae was again observed, and, in addition, egg production per female worm was reduced compared with egg production in previously uninfected sheep given a similar challenge infection.The results of the initial infections suggest that control of the growth and development of H. contortus infection by factors independent of an active host response does not occur in worm burdens established from doses as high as 2700 infective larvae.

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Mesenteric lymph node cells obtained from immunized donors caused the expulsion of transplanted damaged worms of Nippostrongylus brasiliensis in both non-irradiated and irradiated (400 rad) syngeneic recipients (DA strain). They did not cause the expulsion of transplanted damaged worms in syngeneic recipients irradiated at 750 rad, although in one experiment they did suppress worm egg production per female worm. It is clear that both non-irradiated cell recipients and recipients irradiated at 400 rad made an active contribution to worm expulsion. This third component is in addition to the humoral (worm damage by antibody) and cell-mediated components previously defined by the studies of Jones and Ogilvie [1971], Keller and Keist [1972] and Dineen et al. [1973b]. In a collateral study, using the production of plaque-forming cells in rat spleen following antigenic stimulation with sheep red blood cells, it was shown that there was considerable regeneration of immunological reactivity in rats immunized after irradiation at 400 rad during the time course (4–6 days) of the present studies. No such regeneration occurred in rats irradiated at 750 rad. The nature of the third component is currently under investigation.