An infection is the most common cause. It may also be cancer. It would be best to see a doctor who can identify the exact cause.
Lymph protect your body
A lymph node (pronounced /ˈlɪmf noʊd/) is a small circular ball shape organ of the immune system, distributed widely throughout the body and linked by lymphatic vessels. Lymph nodes are garrisons...
Gotta love A&P ..haha The lymph node is a filter so cellular debris, suspended bacteria, other particulates would be removed.(from yahoo)