ABSTRACT
Although we become conscious of infectious agents only when we are suffering from the diseases they cause, microorganisms are always with us. Fortunately, almost all of the microorganisms we come into contact with are prevented from ever causing an infection or disease. To prevent and control infectious disease, the immune system has three lines of defense that operate at different times as an infection progresses. This chapter examines the first line of defense: the mechanisms of innate immunity, which are ready for action at all times and function from the very beginning of an infection. The second line of defense comprises innate immune mechanisms that are mobilized once cells of the immune system have detected the presence of infection and have turned on the gene expression and protein synthesis necessary to make the response. These induced mechanisms of innate immunity, which require from a few hours to four days of development to become fully functional, are the subject of Chapter 3. The third line of defense, the adaptive immune response, is reserved for the small minority of infections that innate immunity cannot subdue. Although slow to mobilize, the adaptive immune response is more powerful than the innate response and has the added advantage of providing long-lasting immunity to further infection by the pathogen. The mechanisms of adaptive immunity are covered in Chapters 4-11 of this book.
