Background Childhood represents an immunological window of vulnerability in which individuals are at increased risk for both serious infections and development of allergic diseases, particularly affecting the airways. However, little is known about how the airway mucosal immune system is organised and functions during early age. Here, the organisation of immune cells in bronchial mucosa of children was characterised.

Methods Immunophenotyping was performed on mucosal samples obtained postmortem from nine children aged 2–15 years without any history of atopic manifestations or any signs of respiratory disease, who died from non-inflammatory causes.

Results In all nine cases, isolated lymphoid follicles (ILFs), interpreted as bronchus-associated lymphoid tissue (BALT), were found, constituting an average frequency of 60 ILFs/cm² of airway mucosal surface. Outside these ILFs, dense networks of CD11c⁺ myeloid dendritic cells (DCs), CD68⁺ macrophages and CD3⁺CD45RA⁻ memory T cells were found. Plasmacytoid DCs occurred in low numbers. Importantly, intraepithelial antigen-presenting cells were found to extend cellular projections into the airway lumen.

Conclusion The density and location of antigen-presenting cells and T cells in this age group are similar to those observed in adults. However, in contrast to adults, BALT appears to be a normal feature of the airway mucosa throughout childhood, suggesting that these structures contribute to regional immunity and homeostasis. This indicates that the local immune system in the airways of children has unique features which should be taken into account, not only when studying airway immunology and immunopathology, but also in the development of mucosal vaccines.