The lymph system is an important part of the body's defense system. It includes lymph nodes, which are small, bean-shaped structures that help filter harmful substances and fight infection.

Breast cancer cells can break away from the original tumor and enter the lymph vessels. These vessels carry fluid throughout the body and connect to lymph nodes, especially those located under the arm — known as axillary lymph nodes.

When cancer cells reach the lymph nodes, it means the cancer is beginning to spread beyond its original location. This is often one of the first steps in cancer progression. Doctors check these lymph nodes to understand how far the cancer may have spread and to decide on the best treatment plan.