1. The titre of anti-Nematospiroides dubius antibody was directly proportional to the degree of protective immunity in mice which indicated that the reactions which protected mice against N.dubius were mediated by antibodies.
2. N.dubius concurrent infections depressed adaptive immune responses in mice. This was evident as reduced specific anti-N.dubius titres and enhanced parasite survival.
3. A threshold existed at which parasite-induced-immunosuppression no longer influenced the expresssion of protective immunity. Above this value the percentage recovery of adult worms decreased significantly irrespective of the mode of infection. The threshold was evident after 4 reinfections.
4. Adult parasites had potent immunosuppressive activity in mice.
5. Secretions from adult N.dubius increased both the survival and reproduction of N.dubius when injected into naive mice.
6. The immunosuppressive secretory factors were unable to depress immunity in previously infected mice.
7. Multiple injections of N.dubius secretions prior to infection immunized, rather than suppressed the immunity of mice. Immunization was progressive; parasite survival declined after 2 and 3 injections and their fecundity was reduced after 3 injections of N.dubius secretions.
8. Injection of fractionated parasite homogenates into mice generally resulted in protection rather than immunosuppression. Immunogenicity was associated with both the AChE active and other protein fractions.
9. Both homogenates and secretions of N.dubius contained cholinesterase (ChE) activity; the secreted enzyme was different from the homogenate enzyme.
10. ChE activity from both sources varied with the immune state of the host.
11. The molecular weight of N.dubius homogenate AChE was 740,000 daltons.
12. Passive protection of mice with immune mouse serum (IMS) was dose dependent. Mice treated with 3 ml were better protected than mice treated with 0.5 ml IMS.
13. Mice infected with 400 N.dubius before passive transfer were protected by 3 but not 0.5 ml of IMS.
14. Serum from donor mice infected with 400 N.dubius, despite high anti-N.dubius antibody titres, was not as protective as serum from mice infected with 50 N.dubius, when passively transferred into naive mice.
15. The albumin fraction of serum from donor mice infected with 400 N.dubius enhanced parasite survival in naive recipients.
16. Immunity to N.dubius had two phases, the first involved the action of specific antibody and the second related to cellular mechanisms.
17. The immunosuppressive factor in IMS 400 serum appeared to inhibit the cellular arm of protective immunity.