Cytokines, chemokines, and growth factors are important mediators that allow communication between cells undergoing differentiation, response to infection, injury, and/or repair. These molecules regulate the pathobiological mechanisms involved in tissue responses to a variety of stimuli. For example, in the lung, infection with bacteria leads to release of chemotactic factors, accumulation of inflammatory cells, release of further cytokines, and stimulation of immune responses, acute phase responses, and subsequent repair responses, all involving multiple cell types, tissues, and body compartments. This cascade of events is meant to stop the injury, wall off invading organisms, initiate the immune defense system, repair and remove the damaged tissue, and return the organism to normal function. Sorting out the role for any one mediator in this complex pattern of events is very difficult, and thus the development of therapeutic interventions remains empiric at best.