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Lymph nodes are part of the body's filtration system. They help protect against infection by trapping foreign agents that flow through them. A buildup of bacteria, viruses, or cancer cells in these nodes can make them swell.

Although lymph nodes are located in many areas of the body, the ones that are most likely to be noticed when swollen are those in the neck, the armpits, the groin, and the crook of the arm. There's no point searching for them when a person is well, because they usually cannot be felt unless they are enlarged.

Such swelling is quite common. Some people experience swelling of lymph nodes in the neck almost as soon as they catch a cold or the flu. Removal of lymph nodes can also cause generalized swelling of the area they normally drain — a condition called lymphedema.

These and most other causes of swollen lymph nodes are short-lived and harmless. However, persistent enlarged lymph nodes, especially accompanied by other symptoms, may be a warning sign of a serious disease.

Causes of enlarged lymph nodes

AIDS
AIDS, or acquired immune deficiency syndrome, is a serious disease in which the causative HIV virus attacks the body's immune system. People with AIDS are vulnerable to a wide array of infections, many of which can cause enlarged lymph nodes. In addition, persons with AIDS are at high risk for a rare cancer, Kaposi's sarcoma, which causes swollen lymph nodes and skin lesions.

Breast cancer
A woman may find a painless ump in her armpit while examining her breasts for signs of cancer. Indeed, breast cancer commonly spreads from the central part of the breast to the lymph nodes in the armpit.

Childhood diseases
Many of the infectious diseases that are common in childhood may provoke swollen nodes and glands in the neck, just below the ear. If the swelling is on only one side of the face, it could be mumps. If the swelling is on both sides and on the back of the neck, and the child also has a rash and a fever, suspect measles (rubeola) or German measles (rubella).

Common infections
Most often, enlarged lymph nodes in the neck are due to common infections such as a cold or flu, resulting in a sore throat and other symptoms. When caused by a virus, no treatment is usually necessary. However, if bacteria is causing the infection, antibiotics are needed to eradicate it.

Drug reactions
Sometimes a generalized allergic reaction to sulfa drugs, iodine, penicillin, or other medications may cause enlarged lymph nodes, which may be particularly noticeable

in the armpit. Other medications, such as those used to treat epilepsy and certain thyroid disorders, sometimes cause swollen nodes.

Lymphedema
Sometimes the lymph channels become blocked, causing an accumulation of lymph in the adjacent body tissue. Swelling also may occur when lymph nodes are destroyed or removed during cancer treatment.

Lymphoma
A lymphoma is a malignant tumor of the lymph glands. One of the most common types is called Hodgkin's disease; the others are referred to as non-Hodgkin's lymphomas. In all types, the first symptoms are most likely to be swollen glands in the neck, armpit, or groin. Other signs may be more vague, such as a generally ill feeling, weight loss, fever, night sweats, and loss of appetite.

Mononucleosis
If glands are swollen in at least two locations and the person also has a fever and fatigue, the cause may be infectious mononucleosis. A doctor should be seen promptly and the patient should take steps to avoid spreading the disease.