A network of lymphatic vessels and lymph nodes drain fluid from the tissue in the breast. The lymph nodes are designed to trap foreign or abnormal cells (such as bacteria or cancer cells) that may be contained in this fluid. Sometimes cancer cells pass through the nodes into the lymphatic vessels and spread to other parts of the body.

Although fluid from breast tissue eventually drains to many lymph nodes, the fluid usually drains first through one or only a few nearby lymph nodes. Such lymph nodes are called sentinel lymph nodes because they are the first to warn that cancer has spread.