What causes a swollen lymph node?

Many things: Many things like- infection ( most common ); inflammation -( trauma, irritation, other systemic inflammation); autoimmune problems, malignancy etc- can cause lymph node enlargement.

Monitoring Centers: The lymphatic system absorbs and carries things too big to go into the arteries and veins like bacteria eaten by white cells and digested food. The lymph nodes "taste" the lymph to see if anything bad is there 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.