The lymph nodes are small rounded organs scattered throughout the body and play a major role in the immune system. A lymph node can swell to more or less impressive sizes, which then forms a visible bulge under the skin if the node is not located too deep. The node can be painful in this case, or tender to the touch, but it can also be painless. When only one or a small localized group of nodes is enlarged, a local cause may be suspected, most often an infection. When many areas of lymph nodes are affected, a more general illness such as mononucleosis, or even cancer or lymphoma should be investigated.