Immunological responses of sheep to Haemonchus contortus

Infections with Haemonchus contortus are a major constraint on ruminant hea lth world-wide. Young lambs are very sensitive to Haemonchus infection. Old er lambs and sheep acquire immunity after a continuous or seasonal exposure to the parasite. The mechanisms underlying immunity are still not complete ly understood. Antibodies, in particular local IgA and IgE, certainly play a role. The role of IgG is less clear. Lymphocyte proliferation responses s eem to correlate ro immunity. Sheep that have high antigen-induced lymphocy te responses have a low susceptibility to infection. Furthermore, several s tudies have demonstrated that immunity against H. contortus associated with mastocytosis and hypersensitivity reactions. More recently, increasing att ention is being paid to the role of cytokines (interleukins and gamma-inter feron) in the activation of specific defence mechanisms. Reverse transcript ase-polymerase chain reaction (RT-PCR) assays to study cytokine mRNA expres sion have become available. The inability of young lambs to mount a signifi cant Th-2 response, which is normally characterized by high IgE levels, mas tocytosis and eosinophilia, may account for the phenomenon of unresponsiven ess in these animals.