[Markers of cell populations in the lower lung]

Trends in basic research in thoracic medicine are increasingly orientated to biochemical and cellular studies. The need for "biochemical markers" as evidence of a particular type of cell is soon appreciated, both from a qualitative as well as quantitative viewpoint. The ideal criteria which these "markers" should possess (specificity for the organ and cell population and reflecting a well defined metabolic activity) are unfortunately rarely satisfied; the "marker" may be analysed in plasma or broncho-alveolar lavage liquid. Two examples are presented and the difficulties of their interpretation are discussed. First in relation to the determination of serum angiotensin converting enzyme activity and then elastolytic activity in the broncho-alveolar liquid. A good knowledge of basic biological phenomena is necessary to take full advantage of the information which can be obtained by measuring "markers" of activity or function of pulmonary cells.