Enlarged lymph nodes
The lymph nodes are small masses of tissue that are part of the immune system. They filter fluids, help in the production and storage of immune cells, and trap harmful cells. These nodes are located in specific parts of the body, including the armpits.
The nodes may become swollen or enlarged when there is an infection in the body. The lymph nodes are not located far from the skin surface so the swelling is often noticeable as a small, painless bump. When a cancer cell moves from the breasts to the lymph nodes near, it can cause them to swell. This is usually noticeable in the armpits or at the nodes near the collarbone.
The lump due to cancer spread into the lymph nodes is different from that due to a cancerous tumor [4]. Although cancerous tumors often develop before the cancer spreads to the nodes, the nodes may enlarge before the tumors are large enough to be felt by the hands.
Enlarged lymph nodes in the underarm are not always a sign of breast cancer. Many infections that have nothing to do with cancer also cause this symptom.