Eosinophils: offenders or general bystanders in allergic airway disease and pulmonary immunity?

Eosinophils have long been noted to be present in asthma and other forms of pulmonary inflammation, but whether they act as true offenders or merely as bystanders has been a point of uncertainty. However, in recent years, there has been increasing evidence suggesting that eosinophils are not passive cells in the respiratory system, acting only as markers of allergic inflammation. This review discusses key evidence from animal models and human clinical trials that support the importance of eosinophils as active and necessary, rather than passive and unnecessary, to the pathogenesis of allergic airway disease. Analyses that are supportive of important immunoregulatory roles of eosinophils in allergic pulmonary inflammation are also reviewed. Data indicating that eosinophils contribute to viral, bacterial, and mycobacterial defense and clearance are detailed. Continually increasing evidence has supported a new conception of eosinophils as being multifaceted immune cells with complex interactions with other immune cells and their local environment.