The ability to understand disease mechanisms and make a correct diagnosis is greatly enhanced by analyzing tissue or body fluid samples; this information complements that obtained by functional tests. This principle has long been applied in nephrology, hepatology and dermatology but was not fully accepted until recently in respiratory medicine. Thus, asthma and chronic obstructive pulmonary disease (COPD) were diagnosed on the basis of history and lung function tests, and the study of pathogenesis was limited by the relative lack of insight into the pathological changes in the airways