Immune system structure and function

Defense against microbes is mediated by the early reactions of innate immunity and the later responses of adaptive immunity. Innate immunity (also called natural or native immunity) provides the early line of defense against microbes . It consists of cellular and biochemical defense mechanisms that are in place even before infection and are poised to respond rapidly to infections. These mechanisms react only to microbes (and to the products of injured cells), and they respond in essentially the same way to repeated infections. In contrast to innate immunity, there are other immune responses that are stimulated by exposure to infectious agents and increase in magnitude and defensive capabilities with each successive exposure to a particular microbe. Because this form of immunity develops as a response to infection and adapts to the infection, it is called adaptive immunity.