Lymph nodes are part of a lymph vessel system that branches throughout the human body. There are about 100 that end up at a point called a lymph node. The lymph vessel system is part of the immune system. It's job is to filter the body of foreign debris and than at some point returns back to the blood stream. Microscopic cancer cells that seed from a primary tumor have the ability to enter the lymphatic system and travel throughout the body and lodge almost anywhere.
Lymph nodes are small, bean sized organs that are vital to our immune system. Each human is born with lymph nodes in the neck, groin, axillae, mediastinum, and abdominal cavities, but the exact number has yet to be counted.