Pathogens that enter the body via mucosal surfaces face unique defense mechanisms that combine the innate barrier provided by the mucus layer with an adaptive response typified by the production and transepithelial secretion of pathogen-specific IgA. Both the measurement and induction of mucosal responses pose significant challenges for experimental and practical application and may need to be adapted to the species under study. In particular, for livestock, immunization procedures developed in small rodent models are not always effective in large animals or compatible with management practices. This paper reviews the latest advances in our understanding of the processes that lead to secretory IgA responses and how this relates to the development of mucosal immunization procedures and adjuvants for veterinary vaccines. In addition, it highlights the complex interactions that can take place between the pathogen and the host's immune response, with specific reference to
Chlamydia/Chlamydophilainfections in sheep.