The arrangement of the chest wall, shoulder bones and muscles, and the upper arm create a small hollow called the armpit. The armpit is the underside of the shoulder joint, and is among the warmest areas of the body. The armpit is also called the axilla.
Blood and lymph vessels serving the arm travel through the armpit. There are more than 20 lymph nodes (small lumps of tissue that are part of the body's lymphatic system, which helps fight infection) in the armpit. These lymph nodes normally cannot be felt through the skin. The armpits have a high concentration of hair follicles and sweat glands.