Immunological unresponsiveness of lambs to infection with Haemonchus contortus. Modulation of responsiveness by exposure to soluble larval antigen.

The in vitro transformation of lymphocytes in the presence of antigen was used to assay the response of lambs to antigens of Haemonchus contortus. Neither uninfected lambs nor lambs infected with H. contortus up to 9 weeks of age developed in vitro lymphocyte responsiveness to antigen. A slight and transient increase in reactivity was demonstrable in some of the infected lambs when these were reinfected with H. contortus at 26 weeks of age. This lack of responsiveness could be modulated through the exposure of lambs, early in neonatal life, to multiple parenteral injections of soluble third stage larval antigen of H. contortus. Thus, antigen specific responses were apparent in the lymphocytes from 21-week-old lambs if these were injected with antigen when 2 to 17 weeks of age. In particular, marked antigen specific reactivity developed in 19- or 24-week-old lambs if such lambs were injected with antigen on five occasions over the first 6 weeks of life and subsequently infected with H. contortus when 7 weeks old. This responsiveness was enhanced when the lambs were reinfected with H. contortus at 26 weeks of age.