Human mucosal membrane surfaces have a distinct protective mechanism that is specifically designed to produce secretory IgA. In the airway, bronchus-associated lymphoid tissue (BALT) is known to be an inductive site of secretory IgA. BALT is a lymphoid aggregate located in the submucosal area of bronchioles, and plays a central role in airway mucosal immunity by inducing the accumulation of secretory IgA-producing cells. Although previously it had been unclear whether BALT is present in the human lung, we demonstrated the expression of BALT in patients with diffuse panbronchiolitis, chronic hypersensitivity pneumonitis, and collagen disease-associated lung diseases. BALT was shown to elevate serum IgA levels and in other ways to have an influence on the symptoms and signs in patients with these pathologic conditions. We also demonstrated that continuous inhaled antigenic stimulation as well as the local production of interleukin-4 and other cytokines promote BALT development, which, in turn, may give rise to BALToma, a type of primary pulmonary mucosa-associated lymphoma.