Brucellosis is one of the most important zoonotic diseases world-wide. This book focuses on host natural resistance and innate immunity against Brucella infection. The impact of Johne’s disease, a chronic, granulomatous enteritis of ruminants and some wild-type species is examined as well. In addition, the molecular and cellular mechanisms of DNA vaccines are addressed in this book, as well as the optimisation strategies that are currently pursued to harness the therapeutic and commercial advantages of DNA vaccination. Moreover, immune responses may substantially differ between conventional laboratory antigens and microbial ones. This book focuses on the experimental pulmonary tuberculosis as a convenient in vivo model to study the disease, providing important advantages over other models. Other chapters examine the defence mechanisms of several flatfish species, which is a main priority to prevent economic losses. The role of the intestine in the immune response in all vertebrates is examined as well, including its cellular components.