The lymphatic system is an essential part of the immune system, which helps the body fight infections or cancers. Whether the lymph nodes contain cancer cells is an important factor when staging breast cancer, determining treatment, and predicting survival. Though breast cancer has the potential to spread to other regions of the body first, it most commonly spreads first to the axillary, or underarm, lymph nodes. This article explains how the lymphatic system works, why the lymph nodes are important in staging breast cancer, when lymph nodes may appear enlarged, and more.