Introduction
Several diseases with similar symptoms result from abnormal infiltration of inflammatory cells into lung tissue. Early in the course of these infiltrative lung diseases, inflammatory cells (white blood cells and macrophages) and protein-rich fluid accumulate in the air sacs of the lungs (alveoli), in the walls of alveoli, and in the spaces between the alveoli (the interstitial space), causing inflammation (alveolitis). If the inflammation persists, the fluid may solidify and scarring (fibrosis) may replace lung tissue.
As alveoli are progressively destroyed, thick-walled cysts (called honeycombing because they resemble the hexagonal cells of a beehive) are left in their place. The condition resulting from these changes is called pulmonary fibrosis. Many diseases can cause pulmonary fibrosis, especially those that involve abnormalities of the immune system.
Although experts in pulmonary medicine think of infiltrative lung diseases separately, they often have similar features. All lead to a decreased ability to transfer oxygen to the blood and all cause stiffening and shrinkage of the lungs, which makes breathing difficult. However, the elimination of carbon dioxide from the blood is usually not a problem.
 See the sidebar Causes of Infiltrative Lung Disease.
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