What causes enlarged lymph nodes under armpit

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Last updated: April 4, 2026

Quick Answer: Enlarged lymph nodes under the armpit, also known as axillary lymphadenopathy, are most commonly caused by infections, particularly those affecting the arm, breast, or chest. They can also be a sign of inflammatory conditions, reactions to medications or vaccines, or, less commonly, cancer.

Key Facts

Infections are the most frequent cause of swollen armpit lymph nodes, accounting for the majority of cases.
Cancer, including breast cancer, lymphoma, and melanoma, is a less common but serious cause of axillary lymphadenopathy.
Reactions to vaccinations, such as those for COVID-19 or influenza, can temporarily enlarge lymph nodes.
Inflammatory conditions like rheumatoid arthritis or lupus can sometimes lead to swollen lymph nodes.
Lymph nodes under the armpit drain fluid from the breast, arm, and upper chest area.

What are Lymph Nodes?

Lymph nodes are small, bean-shaped glands that are part of the lymphatic system. This system is a network of vessels and tissues that helps the body fight infection and disease. Lymph nodes are found throughout the body, including in the neck, groin, and armpits (axillae). They act as filters, trapping viruses, bacteria, and other harmful substances. Immune cells within the lymph nodes then attack and destroy these foreign invaders, leading to a temporary swelling or enlargement of the nodes.

Common Causes of Enlarged Lymph Nodes Under the Armpit

Infections

The most common reason for enlarged lymph nodes under the armpit is an infection. The lymph nodes in the armpit drain fluid from the arm, the upper chest, and the breast area. Therefore, if there is an infection anywhere in these regions, the lymph nodes will likely become swollen as they work to fight off the pathogens. Common infections that can cause this include:

Skin infections: Cellulitis, infected cuts or scrapes, boils, or abscesses on the arm or hand.
Breast infections: Mastitis (infection of the breast tissue), which is common in breastfeeding women.
Upper respiratory infections: While less direct, severe colds or the flu can sometimes cause generalized lymph node swelling, including in the armpits.
Cat scratch disease: A bacterial infection transmitted by cats, often through a scratch or bite.

Inflammatory Conditions

Certain autoimmune and inflammatory diseases can also cause lymph nodes to enlarge. In these conditions, the immune system mistakenly attacks the body's own tissues, which can lead to widespread inflammation, including in the lymph nodes.

Rheumatoid Arthritis: An autoimmune disorder causing chronic inflammation of the joints.
Lupus: A chronic autoimmune disease that can affect various parts of the body.
Sarcoidosis: An inflammatory disease that can affect the lungs, skin, eyes, and lymph nodes.

Reactions to Medications and Vaccines

Sometimes, the immune system can react to certain medications or vaccinations by causing lymph nodes to swell. This is usually a temporary and harmless reaction.

Vaccinations: Vaccines such as the flu shot, MMR (measles, mumps, rubella), or newer vaccines like those for COVID-19 have been known to cause temporary axillary lymphadenopathy. This is a sign that the immune system is responding to the vaccine.
Medications: Certain drugs, including some anticonvulsants, antibiotics, and chemotherapy agents, can rarely cause lymph node enlargement as a side effect.

Cancer

While less common than infections, cancer is a significant concern when lymph nodes are enlarged. Lymph nodes can become enlarged because cancer cells have spread to them from another part of the body (metastasis), or because the cancer originated in the lymph nodes themselves (lymphoma).

Breast Cancer: This is one of the most common cancers that can cause swollen lymph nodes under the armpit, as these nodes drain the breast tissue. A lump in the breast doesn't always accompany swollen lymph nodes.
Lymphoma: Cancers that originate in the lymphatic system, such as Hodgkin's lymphoma and non-Hodgkin's lymphoma, often present with enlarged lymph nodes in various parts of the body, including the armpits.
Melanoma: A type of skin cancer that can spread to nearby lymph nodes.
Leukemia: Cancers of the blood-forming tissues, which can also involve the lymphatic system.

When to See a Doctor

While swollen lymph nodes are often benign and resolve on their own, it's important to seek medical attention if:

The swelling is significant or continues to grow.
The lymph nodes feel hard, fixed (don't move when pressed), or rubbery.
The swelling is accompanied by unexplained weight loss, fever, night sweats, or persistent fatigue.
The swelling persists for more than two weeks without an obvious cause like a recent infection.
You notice other concerning symptoms like a lump in the breast or changes in the skin.

A doctor can perform a physical examination, ask about your medical history, and may order tests such as blood work, imaging (ultrasound, mammogram, CT scan), or a biopsy to determine the cause of the enlarged lymph nodes.