Neutrophils are an important component of innate immunity in the lungs. During bacterial pneumonia, neutrophils are recruited from the capillaries of the pulmonary circulation in the gas-exchanging regions of the lungs. This process requires the coordinated activation of many cells within the lungs, including neutrophils and capillary endothelial cells. Cellular activation during innate immune responses is mediated in part by tumor necrosis factor-alpha (TNF-alpha) and interleukin (IL)-1-initiated signaling through their receptors, activation of nuclear factor kappa B (NF-kappaB) and downstream gene transcription, endothelial cell signaling initiated by neutrophil adherence to intercellular adhesion molecule (ICAM)-1, and binding of leukocyte adhesion molecules to cellular and matrix ligands. These events are essential to effective host defense during pneumonia.