The Immune System: A Very Short Introduction describes the immune system and how it works in health and disease. It focuses on the human immune system, considering how it evolved, and the basic rules that govern its behaviour. The immune system comprises a series of organs, cells, and chemical messengers that work together as a team to provide defence against infection. These components are discussed along with the critical signals that trigger them and how they exert their protective effects, including innate and adaptive responses. The consequences of too little immunity (immunodeficiency), caused for example by HIV/AIDS, and too much, leading to auto-immune and allergic diseases, are also considered.