Lymph nodes are intricately involved in your body's immune response, contributing to your ability to fight off infections. Located in various regions of your body -- including behind your ears -- nodes fill with fluid when you are exposed to a bacteria or virus.

Lymph nodes are a part of your immune system. The immune system protects you from infection and other diseases. It includes your spleen, bone marrow, thymus, tonsils, adenoids, the lymph channels and the lymph nodes.