The Progression of Infiltrates in Pneumonia

In the early stages of bacterial pneumonia, the infiltrate seen on chest X-ray consists mainly of neutrophils and bacteria, along with other components like edema and fibrin. This inflammatory infiltrate, or pus, is a result of the immune response to the invading bacteria. After the bacteria are cleared, the neutrophils are replaced by macrophages, which engage in efferocytosis, the process of removing apoptotic bodies. This cleanup process by the macrophages continues for weeks, even after the patient has clinically stabilized.