Lymph nodes produce immune cells to help fight infection. They also filter the lymph fluid and remove foreign material, such as bacteria or cancer. Common areas where lymph nodes can be felt include the armpit, behind the ears, and sides of the neck.
When breast cancer spreads, lymph nodes in and around the armpit are some of the first places it travels, and surgeons often remove some of these nodes to determine whether the cancer has spread.
After lymph nodes in the armpit have been removed, lymphedema, or swelling of the arm, can occur. Lymphedema happens because, after surgery, there are no longer enough nodes to process the lymph, so the flow of it gets backed up in the incision area.