Asthma is a complex chronic inflammatory disease of the airways that involves the activation of many inflammatory and structural cells, all of which release inflammatory mediators that result in the typical pathophysiological changes of asthma (Barnes, 1996a)(table 1). By inflammatory mediators, we mean cell products that are secreted and exert functional effects. We reviewed the mediators of asthma in 1988 (Barnes et al., 1988), but since then there have been major advances in our understanding of the mechanisms of asthma and the role of inflammatory mediators. There is now greater understanding of each mediator; in addition, novel mediators of asthma, such as the cytokines, have been identified. To date, 50 different mediators have been identified in asthma. Advances in this field have been greatly assisted by the development of potent and specific inhibitors that either block the inflammatory mediator receptors or inhibit mediator synthesis.

In writing this review, we have focused on new developments since 1988 and have emphasized studies in humans wherever possible. There is a vast and rapidly increasing body of literature on mediators of asthma; therefore, we have been forced to be somewhat selective. We have chosen to emphasize the mediators and effects that we think are most relevant to human asthma.