A diagram of a lymph node, courtesy KC Panchal, via Wikipedia
The lymph nodes are part of the lymphoid system, which regulates fluid, fat, and the immune system. The nodes act as filters, removing foreign material from the lymphoid systems and triggering the production of antibodies when an infection that a person has already suffered when it recognizes such an infection.
Although the lymphoid system can be found throughout the body, the nodes are in more or less the same location in every living person. As such, they can provide important diagnostic clues. When the body is fighting off an infection, the nodes often become swollen as they swell with fluid. As such, physicians routinely check the lymph nodes that are close to the skin (such as those in the upper neck and the armpits) to determine whether a patient is suffering from an infection.