Lymphatic vessels are similar to veins, but carry lymph instead of blood.
Lymph is a clear fluid that contains tissue waste products and immune system cells. Cancer cells may enter lymph vessels and spread out along these vessels to reach lymph nodes. Cancer cells may also enter blood vessels and spread through the bloodstream to other parts of the body. When breast cancer cells reach the axillary lymph nodes, they can continue to grow, often causing swelling of the lymph nodes in the armpit. If breast cancer cells have multiplied in the axillary lymph nodes, they are more likely to have spread to other organs of the body as well.