BACKGROUND--Bronchus associated lymphoid tissue (BALT) is a normal component of the lung's immune system in many animals and may be analogous to gut associated lymphoid tissue (GALT). This study aimed at assessing the nature and extent of BALT in human lung and determining whether its expression is induced within the human airway in response to smoking. METHODS--Paraffin embedded, formalin fixed full thickness bronchial wall sections were examined from 31 whole lung specimens derived from both smokers and non-smokers. Samples were taken from throughout the bronchial tree to include main stem bronchi, lobar bronchi and segmental bronchi, as well as first to third generation carinae. Standard 4 microns step sections were stained by haematoxylin and eosin and immunocytochemical methods to show foci of BALT. RESULTS--Examination of 256 airway sites detected 46 foci of BALT. These differed from those described in other mammals in being distributed throughout the bronchial tree, in being found in relation to bronchial glandular epithelium as well as luminal bronchial epithelium, and in lacking any accompanying M cells. Analysis by smoking status showed that the expression of BALT was significantly more common in smokers than non-smokers (82% (14/17) v 14% (2/14) respectively). CONCLUSIONS--The findings support the view that BALT in humans is an integral feature in a comparatively small proportion of lungs from non-smokers while being significantly more prominent in lungs from smokers. The tissue shows several important differences from that described in other mammals.
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