The document discusses the immune system, which protects the body from disease. It has an innate immune system with physical barriers and cells like phagocytes and natural killer cells that are always present to fight microbes. If pathogens evade this innate response, vertebrates have an acquired immune system activated by the innate response. The acquired immune system adapts its response during infection to improve pathogen recognition, retain immunological memory, and launch faster, stronger attacks if the pathogen is encountered again.