In the respiratory tract, lymphoid aggregates with a specialized epithelium have been called bronchus-associated lymphoid tissue (BALT) and compared to the organized lymphoid tissue of the gut (GALT), e.g., Peyer's patches. BALT might play a central role in antigen uptake, initiating immune responses and disseminating primed lymphoid cells in the respiratory tract. In the present study, lungs of mice, rats, guinea pigs, rabbits, pigs, cats, and humans have been studied with respect to the presence and number of BALT and the dependence of BALT on age and microbial stimulation. BALT is not a constitutive structure in all these species. Its frequency varies widely, from 100% in rabbits and rats, 50% in guinea pigs, 33% in pigs, to its absence in cats and all normal human lungs. BALT seems to be a lymphoid structure which is not present in all the species studied but can develop in the lung after stimulation. This is in contrast to lymphoid organs, such as lymph nodes or Peyer's patches, which can always be found. These species differences are of major importance in interpreting the clinical relevance of experiments in animal models on the lung immune system, e.g., antigen uptake, immunostimulation, or lung transplantation.
The epithelium associated with lymphoid aggregates in the bronchial tract (BALT) was studied in rabbits by immunohistochemistry using monoclonal antibodies against the secretory component (SC) of IgA. The normal bronchus epithelium was intensely labelled. In contrast, epithelium overlying the central parts of the follicles was negative. This specialized epithelium cannot participate in the SC-mediated transport of IgA, which might be a basis for the adherence and transport of microorganisms into the lymphoid tissue, thus initiating immune responses of the BALT.
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