Leukocytes of the innate immune system play a central protective role in immune defense to pathogens but may also mediate injurious inflammatory responses resulting in tissue injury. These leukocytes provide the first rapid cellular defense mechanisms through a limited repertoire of rapid pre-programmed responses, but they are also involved in chronic inflammation and tissue repair. They are directed to sites of pathogen challenge and inflammation by a variety of mechanisms and are activated in response to both exogenous and endogenous stimuli. They do not show the capacity of self-non-self discrimination and memory, which are defining characteristics of the adaptive immune system, although macrophages in particular may show some capacity for differentiation of their effector responses. However, they do play an integral role in adaptive immune responses by their capacity to present antigen, modify T-cell development, and function as effectors of adaptive cell-mediated immunity.