Bronchus-associated lymphoid tissue: a review of its development and function, including recent findings on the impact of environmental particulate exposure.

Bronchus-associated lymphoid tissue (BALT) influences the mucosal immune system, specifically lung immunity. In many animal species, BALT is found in normal lung tissue, whereas in others, including humans, it is rarely present under normal conditions but it develops after exposure to atmospheric pollutants or allergens, during viral or bacterial infections, or autoimmune diseases, known as inducible BALT (iBALT). The morphology and function of BALT depend on exposure to specific antigen types; however, the underlying mechanisms are unclear. Here, we review available data, including our recent research, on BALT development and function, with the aim of promoting studies on the immune system. In this review, we first present our recent findings from experiments using mice on the induction of BALT formation by environmental particles, and then provide an overview of the structure and function of BALT. We subsequently discuss the key cellular components involved in BALT formation under various pathological conditions, with a particular focus on immune cells. Finally, we introduce the dual roles of BALT, highlighting its protective and pathogenic effects. BALT induction is one of the most important mechanisms underlying pulmonary antigen-specific immunological defence and is, therefore, expected to become an increasingly important target for immune system research into various lung diseases, including diseases caused by environmental particulates.