ABSTRACT – Mariana Gomes Nogueira Lawsonia intracellularis is the causative agent of proliferative enteropathy (PE). PE is an important disease of weaner and grower pigs causing degrees of diarrhoea and negative effects on feed intake and weight gain. In-feed antibiotics are routinely used to control PE disease outbreaks. However, increasing restrictions to antibiotic use is being implemented in various pig producing countries. Therefore, alternatives to improve resistance while promoting growth performance are ideal. To limit the establishment of infection and increase profitability of pig production, disease prevention needs both nutritional and immunological strategies and well as effective sanitary measures. Since 2006, in the Australian market, a live attenuated Lawsonia intracellularis vaccine has been used to reduce clinical signs and PE lesions and reduce L. intracellularis shedding in faeces. However, the systemic and local immunological responses to a standard vaccine dose are poorly characterised. In the absence of proof that vaccinated pigs are protected from PE, veterinarians are unwilling to remove antibiotic medication for fear of acute haemorrhagic PE in adult pigs. Additionally, feeding strategies as alternatives to antibiotics, such as beta-glucan have been proposed as a possible option, but no study has investigated the effect of phytase on immune responses in growing pigs. The research in this thesis addresses studies to immune-based investigation of factors affecting the induction of immune responses following vaccination with L. intracellularis. Results revealed that the use of an oral standard and ten times dose of L. intracellularis vaccine protects pigs against PE disease by reducing lesions, shedding in faeces and clinical signs. However, intramuscular delivery also protected pigs against PE. Immunological responses to L. intracellularis vaccination, particularly IgG and cytokines response were observed after oral, intraperitoneal and intramuscular L. intracellularis vaccination. However these were highly variable, highlighting the difficulties in finding suitable biomarkers. The effect of adding S. cerevisae yeast beta-glucan and microbial phytase to weaner diet affected on mucosal and systemic L. intra