The lymphatic system serves as the sewage system for cellular waste in the body. The lymph vessels follow closely beside the blood vessels and receive the cells' waste products. This waste is carried by the lymphatics and is filtered through rounded areas of the lymph system referred to as the lymph nodes. Nodes appear as small round capsules and vary in size from several millimeters to several centimeters in diameter. Lymphocytes and monocytes are produced in the nodes and act as filters to stop bacteria, cellular waste and cancer cells to keep them from entering the blood stream. The lymph nodes may also serve as placed where cancer cells set up metastatic sites ( cancer that spreads from the original site to nodes).

Three percent of the lymphatic fluid draining the breast is found in the lymph nodes located in the area of the breast bone in the internal mammary nodes. Ninety seven percent of the fluid is drained through the nodes located in the area of the armpit, referred to as the axilla.

There are three levels of lymph nodes under the arm. The first level is located from the breast to the underarm area; the second level is behind the pectoralis minor, a small muscle on the chest wall. And the third level is located higher on the chest near the collar bone.

Axillary dissection, which is performed on the majority of patients with invasive carcinoma of the breast, refers to the removal of nodes from different levels of the axilla. The number of nodes vary from person to person.