The survival of each animal species depends on its ability to recognize invading pathogens and respond to them rapidly. Many such defenses against microbes are innate rather than adaptive to the particular pathogen. Just as infants instinctively respond to the auditory and visual cues of their parents, so the immune system is programmed to recognize biochemical patterns displayed by microbial pathogens and mount rapid responses to them. The innate immune system includes macrophages and natural killer cells, which may act directly on the pathogen or, by releasing cytokines and expressing certain other stimulatory molecules, trigger adaptive immune responses by activating . . .