Figure 2.

LOWER RESPIRATORY TRACT. (A) BALT follicular anatomy in lungs: Lymphocytes home to the bronchus subepithelial space via lymphatic vessels and HEVs where they form an ectopic lymphoid structure known as the bronchus associated lymphoid tissue (BALT). This tissue is characterized by densely packed B cell follicles with distinct germinal center (GC) behavior. (B) Outcomes of BALT induction: The BALT plays a positive protective role against respiratory pathogens via antibody (IgA, IgG) clearance. However, during prolonged chronic inflammation, BALT is associated with harmful conditions such as autoimmunity and cancer. (C) Timeline of BALT formation and degradation in humans: Infant BALT often forms in fetal lungs, reaches maximum size by year two to three, and fully degrades by year four or five. As an adult, BALT can be transiently induced by respiratory challenge but will degrade following clearance and resolution. However, chronically induced BALT that persists over time is a marker for harmful chronic immune activation and inflammation in the bronchials. (Created with BioRender.com).