ABSTRACT
Respiratory tract infections, including pneumonia, are among the commonest causes of death worldwide. Contaminated oropharyngeal secretions are often aspirated into the lower respiratory tract. Although aspiration of oropharyngeal secretions is common, the host defense mechanisms are usually able to clear potential pathogens and thus prevent parenchymal infections. A significant part of normal host defenses against pulmonary infection relates to the anatomical and mechanical barriers provided by the upper respiratory tract. From the nasopharynx to the distal terminal bronchiole, the inner surface of the respiratory tract is lined with a pseudostratified ciliated epithelium interspersed with secreting cells (goblet cells) and other cells such as the dendritic cells. The respiratory epithelial cells are also the source of powerful antimicrobial peptides such as the cathelicidins and human B defensins that can act as chemokines at the level of the T lymphocytes and dendritic cells.
