This chapter focuses on the immune and lymphatic systems and the nurse's role in the recognition and management of patients who are at risk of acute deterioration due to altered immunity and sepsis. It reviews the important cellular and chemical components that seek and destroy invading micro-organisms in order to protect the body from infection. This coordinated response is the role of the immune and complement systems and in combination with the body's natural defences protects from a multitude of microbes. The chapter further describes the spread of microbes and the role of health care professionals in actively preventing the spread of health care associated infections (HCAIs). The role of the nurse is crucial in recognising the early signs of infection and in the delivery of evidence-based care to those who experience acute deterioration from problems related to immunity and infection. It explores the clinical problems that lead to a medical emergency of anaphylaxis and sepsis.