Using a simple and graphical approach this atlas addresses the mechanisms involved in the immune responses of dogs and cats, and describes the most important infectious and parasitic diseases, as well as the corresponding preventive strategies. A particular emphasis is placed on pathological immune processes such as allergies, dermatitis, and possible adverse reactions to transfusions and vaccinations. This atlas is designed as an aid to veterinary surgeons who wish to explain these important and complex concepts to pet owners in simple but precise terms. This will help owners understand the processes underlying the development of some of the most important diseases of their pets, the immune defence mechanisms involved, and the importance of disease prevention through immunisation, ultimately aiding decision-making in specific cases.