This course aims to provide an understanding of the components (organs, cells and molecules) of the vertebrate immune system and their mood of action. The lectures will focus both on mammalian species and on bony fish. In the general part, the course will particularly discuss on how the immune system protects the body from infectious microorganisms like virus, bacteria, fungi and parasites. The student will learn how immune responses, including both innate and adaptive responses, are initiated and terminated, and how the immune system "remembers" a pathogenic intruder such that it can respond stronger at subsequent infections (the basis for vaccination). In the fish immunology part, an overview of the immune system in fish, with emphasis on aquaculture species, is provided.