Figure 1.

Mucosa-associated lymphoid tissues. The immune system associated with various mucosal surfaces constantly surveils these points of entry for pathogens. Humans do not have a defined bronchi-associated lymphoid tissue (BALT), but rather the BALT is induced upon infection. The upper and lower respiratory tract mucosa includes undifferentiated basal cells, ciliated epithelial, pulmonary neuroendocrine (PNEC), and secretory goblet cells. The mucus secreted by the goblet cells provides a barrier to debris, allergens, and potential pathogens but also to mucosal vaccines. Created with BioRender.com.