Why do lymph nodes swell up?

Immune System HQ. The lymph nodes can be thought of as outposts for white blood cells. When there is an infection or inflammation in the area where a lymph node is draining fluid from, the white blood cells start to build up to fight it. This causes swelling in that node.
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.