How serious are lymph node enlargements?

Depends on cause: Lymph nodes help to fight off a variety of illnesses, including infections, non-infectious inflammations, cancer, etc. The severity depends on what caused them to enlarge.

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.