In contrast, circulating IgA levels in control wool lambs remained low and stable following initial larval exposure, and breed differences in circulating IgA were thus larger in control lambs. Greater responses in circulating IgA in response to transient exposure to parasite larvae in control hair lambs suggests a more robust response to this ‘natural’ vaccination protocol. IgA concentrations in hair sheep were somewhat higher than those reported for wool sheep in previous studies. Larval antigen-specific IgA production has been reported to peak between 1 and 2 weeks p.i. (42,44). However, total IgA in serum in our infected lambs continued to increase through 3 weeks

p.i. in both breeds. Previous studies have not observed higher serum IgA concentrations in resistant breeds including the Gulf EPZ-6438 mouse coast native (40), Santa Ines (17) and Barbados Blackbelly (34) compared with susceptible wool breeds, which may indicate that St. Croix hair sheep have a novel resistance mechanism that is absent in other resistant breeds. Globule leucocytes are described as partially

de-granulated intraepithelial mast cells (45) and have been suggested to be responsible for larvae damage and expulsion within the first few days of infection. Unlike eosinophils (46,47), mast cells bind IgE (41), leading to similar co-dependency to that suggested for eosinophils BMN 673 cell line and IgA. Breed differences in abomasal globule leucocytes were not significant in this study, but levels tended to be greater in hair sheep at 27 days p.i. This result is less striking than the 15- to 40-fold increase in globule leucocytes of younger infected hair compared with wool lambs reported by Gamble and Zajac (18). However, breed differences in concentration Protein kinase N1 of globule leucocytes have been reported to be minimal by 1 year of age (3) and hence age differences probably contribute to apparent inconsistencies among studies. Associations of increased globule leucocytes with lower FEC, lower worm numbers and decreased female worm length are present in young animals (11,15,48), suggesting a role for these cells in resistance and are consistent with our favourable association of globule

leucocyte numbers with IgE in the lymph nodes and PCV at 21 days p.i. Measurement of sheep IgE was first reported by Shaw et al. (49) and Kooyman et al. (8) and several studies have shown that sheep infected with GIN have increased total and worm-antigen-specific IgE (8,12,13,50,51). Mean serum IgE levels in our lambs exceeded 60 ng/mL through 16 days p.i. in infected lambs of both hair and wool types. IgE levels were similarly elevated through the first 16 days following exposure and subsequent de-worming in control hair lambs, but were only transiently elevated in control wool lambs over the same period. These patterns suggest a similar change in circulating IgE following infection in the two breeds with a potentially more robust vaccination response in hair lambs, similar to that observed for circulating IgA.