In order to include immunocompetence in selection for improved health, a major challenge will be to correlate variation in heritable ITs in healthy animals with inter-individual variability in response to various pathogens. Testing this hypothesis will be a key point for further use of ITs as indirect selection criteria in multitrait selection to improve resistance to disease. Some results on genetic and phenotypic relationships between immunocompetence and susceptibility to specific pathogens have already been reported in the literature for pigs. Among pigs selected for eight generations for high (HIR) or low (LIR) response based on an index of four cell and humoral-mediated immunity traits, an increased specific antibody response and lower polyserositis were observed in the HIR pigs compared to the LIR pigs after challenge with a novel pathogen, Mycoplasma hyorhinis [12, 37] . Thus, animals with a high IR level to unrelated challenges, as defined by Wilkie and colleagues [12, 37] , have a better response to infection with Mycoplasma hyorhinis. However, HIR pigs develop more severe arthritis than LIR pigs. Indeed, the levels of humoral and cell-mediated adaptive ITs included in the Wilkie et al index induce the formation of immune complexes and/or the development of inflammatory responses, central to the pathogenesis of Mycoplasma hyorhinis-induced arthritis. Other correlation tests between ITs and response to various infections are needed. However, it is important to remain cautious with high responder animals, which could develop autoimmune pathologies or pathological iummne responses.