The bean-shaped lymph nodes of the lymphatic system are connected by vessels. Lymph nodes are usually present in clusters in the armpits, on either side of the neck, and in the groin. The lymph nodes filter lymph fluid and trap foreign materials. Any fluid absorbed by the lymphatic system passes through at least one lymph node before it returns to circulation. The lymph nodes contain lymphocytes (white blood cells) which help destroy foreign bacteria or other harmful cells. The lymph nodes may become enlarged or swollen when they fight an infection since they must produce additional white blood cells. The lymph nodes may feel tender or inflamed as they are actively fighting a foreign body. Sometimes, the lymphatic vessels will become visible as thin red lines along a limb as the result of an infection (known as lymphangitis). Lymph nodes may also swell from the formation of an abscess (closed pocket filled with pus) in the nodes or if they contain cancer cells.