Lymphoid tissue in human lung. (A): Bronchus-associated lymphoid tissue (BALT). There is a well demarcated submucosal lymphoid follicle in the wall of a bronchus. The arrow is in the bronchial lumen, pointing to the bronchial epithelia. (B): Nonspecific lymphoid aggregate. There is a small, poorly demarcated lymphoid aggregate in a small focus (<1 mm diameter) of scar tissue in the lung interstitium. The aggregate is not associated with a bronchus. This aggregate does not meet the histologic criteria for BALT and is not included in this study. (hematoxylin and eosin stained frozen sections; original magnification 200×).