The breast also has a lymphatic system, which is an essential part of the immune system in filtering harmful substances and fighting infections or cancers.
The lymphatic system consists of bean-shaped lymph nodes (the main ones are in the neck, armpits and groin) which are linked by vessels that contain lymph. Lymph flows around the body in these vessels in the same way blood flows in the veins and arteries. The lymph nodes filter lymph fluid and trap foreign materials. Any fluid absorbed by the lymphatic system passes through at least one lymph node before it returns to circulation. Lymph is a clear whitish/yellowish fluid that contains white blood cells (lymphocytes), proteins, and some red blood cells.
The lymph nodes contain lymphocytes which help destroy foreign bacteria or other harmful cells. The lymph nodes may become enlarged or swollen when they fight an infection since they must produce additional white blood cells. Sometimes, the lymphatic vessels will become visible as thin red lines along a limb as the result of an infection (known as lymphangitis). Lymph nodes may also swell from the formation of an abscess (closed pocket filled with pus) in the nodes or if they contain cancer cells.
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 (underarm) lymph nodes. This is known as the regional spread. From there, the breast cancer can (spread) systematically to other areas of the body (such as the bone, liver, lung, or brain).