For a quick overview of the many different types of breast cancer, read this brief article. Breast cancer is a malignant tumor that has developed from cells of the breast. The disease occurs mostly in women, but does occur rarely in men. The remainder of this document refers only to breast cancer in women. A separate document on male breast cancer is also available from the American Cancer Society.
Lymphatic vessels are similar to veins, except that they carry lymph instead of blood.
Lymph is a clear fluid that contains tissue waste products and immune system cells that are on their way from areas of infection. Cancer cells can enter lymph vessels. Lymph nodes are small bean-shaped collections of immune system cells that are important in fighting infections. When breast cancer cells reach the axillary lymph nodes, they can continue to grow, often causing swelling of the lymph nodes in the underarm area. If breast cancer cells have grown in the axillary lymph nodes, they are more likely to have spread to other organs of the body as well.