What causes enlarged lymph nodes of the lungs?

Inflammation: Enlarged lymph nodes occur when the immune system is active in a part of the body (a regional factory and distibution site for inflammatory cells which travel from the bloodstream and into the lymphatic fluid). Sources of inflammation in the lungs include infection, autoimmune disorders, injury of lung tissue, chronic respiratory inflammation, and malignancies.

Monitoring Centers: Lymphatic system absorbs/carries things too big to go into the arteries/veins like bacteria eaten by white cells and digested food. Lymph nodes "taste" the lymph for anything bad like bacteria/viruses/cancers, etc. When it detects a problem, it alerts the immune system and your body reacts. They enlarge in response to the inflammation they create when they react. They shrink when all is well.