Pathophysiology of pneumonias
• Bacteria from the upper airways via aspiration or, less commonly, from other sites via hematogenous spread, find their way to the lung parenchyma
• Once there, combination of host and bacterial factors may lead to pneumonia
• Depending on pathogen, the resultant immune response will be predominantly Th17 (most extracellular pathogens) or Th1 (most intracellular pathogens)
• Th17: acute inflammation resulting in the migration of neutrophils out of capillaries and into the air spaces; exudate, dead cells/debris result in alveolar filling and pus
• Th1: acute inflammation bringing NK, CD8, macrophages to alveolar septae, resulting in thickening (no migration into alveoli necessary, no significant exudate)