Immune responsiveness of Romney sheep selected for resistance or susceptibility to gastrointestinal nematodes: field studies.

Long-term selection of sheep for resistance to parasite infections may be jeopardized if animals do not retain their normal ability to respond to non-parasite antigens. Therefore the antibody responses to ovalbumin (OVA) and human red blood cells (HRBC), and kinetics of peripheral blood lymphocyte phenotypes were examined in mature grazing sheep, genetically resistant or susceptible to gastrointestinal nematodes. In both lines the HRBC antibody response peaked 2 weeks after the primary injection, 1 week after the second injection and 3 weeks after the second OVA injection. The antibody titres of the resistant line sheep decreased sooner after both primary and secondary injections. The resistant line sheep had higher percentages of CD5+ and CD4+ cells than the susceptible sheep. Two injections of OVA and HRBC did not result in significant alterations in percentages of CD5+, CD4+, CD8+ and CD45R+ lymphocytes in either line. In both lines, the control groups showed a steady increase of 0.29% per week in percentages of T19+ (gamma delta) T cells which was significantly higher than in the antigen injected sheep.