The most common inflammatory diseases of the airways are asthma and chronic obstructive pulmonary disease (COPD), and the incidence of both is increasing throughout the world. The prevalence of asthma in developed countries is approximately 10% in adults and even higher in children, whereas in developing countries, the prevalence is lower but increasing rapidly. The global prevalence of COPD is approximately 10% among individuals over the age of 40 years. Asthma often starts in early childhood and is characterized by intermittent wheezing and shortness of breath. In contrast, the predominant symptom in COPD is shortness of breath on exertion, which is persistent and slowly progressive. Although the clinical symptoms of both diseases are caused by airway narrowing as a result of inflammation in the airways, there are marked differences in the patterns of underlying inflammation.
Cytokines play a key role in orchestrating the chronic inflammation of asthma and COPD by recruiting, activating, and promoting the survival of multiple inflammatory cells in the respiratory tract (Figure , Figure , and Table ). Over 50 cytokines have now been identified in asthma and COPD, but their role in the pathophysiology of these complex airway diseases is often unclear. For the purpose of this Review, cytokines are classified into lymphokines (cytokines that are secreted by T cells and regulate immune responses), proinflammatory cytokines (cytokines that amplify and perpetuate the inflammatory process), growth factors (cytokines that promote cell survival and result in structural changes in the airways), chemokines (cytokines that are chemotactic for inflammatory cells), and antiinflammatory cytokines (cytokines that negatively modulate the inflammatory response), although many of these functions may overlap. Where possible, I have focused on human studies and have emphasized any therapeutic interventions that have been made.
Key cytokines involved in asthma and COPD