Your lymphatic system is a network of organs, vessels and lymph nodes situated throughout your body. Many lymph nodes are located in your head and neck region. Lymph nodes that frequently swell are in this area, as well as in your armpits and groin area.

Swollen lymph nodes are a sign that something is wrong somewhere in your body. When your lymph nodes first swell, you might notice:

• Tenderness and pain in the lymph nodes

• Swelling that may be the size of a pea or kidney bean, or even larger in the lymph nodes

Depending on the cause of your swollen lymph nodes, other signs and symptoms you might have include:

• Runny nose, sore throat, fever and other indications of an upper respiratory infection

• General swelling of lymph nodes throughout your body — which may indicate an infection, such as HIV or mononucleosis, or an immune disorder, such as lupus or rheumatoid arthritis

• Hard, fixed, rapidly growing nodes, indicating a possible tumor

• Fever

• Night sweats

A lymph node is a small, round or bean-shaped cluster of cells covered by a capsule of connective tissue. The cells are a combination of lymphocytes — which produce protein particles that capture invaders, such as viruses — and macrophages, which break down the captured material. Lymphocytes and macrophages filter your lymphatic fluid as it travels through your body and protect you by destroying invaders.

Lymph nodes are located in groups, and each group drains a specific area of your body. You may be more likely to notice swelling in certain areas, such as in the lymph nodes in your neck, under your chin, in your armpits and in your groin. The site of the swollen lymph nodes may help identify the underlying cause.

The most common cause of swollen lymph nodes is an infection, particularly a viral infection, such as the common cold. Other possible causes of swollen lymph nodes include:

Common infections

• Strep throat

• Measles

• Ear infections

• Infected (abscessed) tooth

• Mononucleosis

• Skin or wound infections, such as cellulitis

• Human immunodeficiency virus (HIV) — the virus that causes AIDS

Uncommon infections

• Tuberculosis

• Certain sexually transmitted infections, such as syphilis

• Toxoplasmosis — a parasitic infection resulting from contact with the feces of an infected cat or eating undercooked meat

• Cat scratch fever — a bacterial infection from a cat scratch or bite

Immune system disorders

• Lupus — a chronic inflammatory disease that can target your joints, skin, kidneys, blood cells, heart and lungs

• Rheumatoid arthritis — a chronic inflammatory disease that targets the tissue that lines your joints (synovium)

Cancers

• Lymphoma — cancer that originates in your lymphatic system

• Leukemia — cancer of your body's blood-forming tissue, including your bone marrow and lymphatic system

• Other cancers that have spread (metastasized ) to lymph nodes

Other possible but rare causes include certain medications, such as the anti-seizure medication phenytoin (Dilantin) and preventive medications for malaria.

If infection is the cause of your swollen lymph nodes and isn't treated, these complications might occur:

• Abscess formation. An abscess is a localized collection of pus caused by an infection. Pus contains fluid, white blood cells, dead tissue and bacteria or other invaders. An abscess may require drainage and antibiotic treatment.

• Bloodstream infection (bacteremia). A bacterial infection anywhere in your body can progress to sepsis, which is an overwhelming infection of the bloodstream. Sepsis may progress to organ failure and death. Treatment involves hospitalization and intravenous antibiotics.