An accumulation of lymphoid cells with a typical localization of B lymphocytes preferentially in a follicle and T lymphocytes, more peripherally around high endothelial venules in the wall of bronchi, is called bronchus-associated lymphoid tissue (BALT). A further structural component is a cap-like accumulation of lymphoid cells partly bulging into the lumen of the bronchus, called the dome area. The epithelium covering the dome lacks goblet cells, is infiltrated by lymphocytes, and contains cells specialized for antigen uptake-M cells. BALT is not present in all species and age groups and can be classified as a tertiary lymphoid organ. A hypothesis is proposed for a two-step vaccination protocol: first, BALT is induced and activated, and second, an antigen-the vaccine-is applied locally. BALT is part of the integrated mucosal-associated lymphoid tissue system.