The lymph nodes, which are located in many areas of the body including the neck, the armpit, the groin, under the jaw, the back of the head and other places, produce immune cells that help the body fight infection. Because they help to filter cancer out of the body, they may become cancerous if there is a mass nearby that is attempting to spread through the body.
Lymph nodes may be swollen when fighting bacteria or disease. When they become swollen and tender, their location is far more obvious, leading many to say that their "glands are swollen".
A sentinel lymph node is the closest lymph node or nodes to a known cancer site. It is the most likely node to be affected by cancer. If the node is positive for cancer, this is an indication that the cancer is attempting to spread through the body.