My major interest is in understanding and controlling nematode infections. Most of my research is on Teladorsagia circumcincta infections of sheep. This host-parasite association was chosen deliberately because it is an economically-important infection of sheep. Most advances in our understanding of host-parasite relationships have come from a detailed analysis of deliberate infections or mathematical modelling of natural infections. Both strategies can be used to study parasites of sheep. The plan is to develop a coherent explanation that covers all levels from populations to molecules. Among the achievements are the use of variance analysis to identify and quantify the major components of resistance, the demonstration that many components including the heritability and sex differences vary with age, the finding that a major mechanism of resistance in growing lambs is the mucosal IgA response against fourth-stage larvae which is associated with reduced worm growth and fecundity and the identification of two quantitative trait loci (QTL) for parasite resistance; one in the mhc class II region and one in or around the interferon gamma gene. Our current research aims to identify the parasite molecules recognised by resistant but not susceptible hosts.