Lymph nodes under the arm are known as axillary lymph nodes, and most people have between 20 and 30 of them. These nodes circulate fluid called lymph throughout the body and toward the heart, hence the large amount of them in the chest and underarm area.
Axillary lymph nodes are commonly involved in breast cancer because of their proximity to breast tissue. Involvement of the lymph nodes makes it more likely that cancer spreads to other locations. Lymph node swelling is common. Non-cancer reasons for swelling can include injury, infection, colds and viral infections, such as measles, mumps or mononucleosis. Side effects from medications, such as phenytoin, can also cause swelling.