Swollen Lymph Node... or Cancer?

Patient Expert
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What do underarm lymph nodes do? Why do they swell up? And how do you know if the swelling is simply an infection – or might be cancer?

You're taking a shower, soaping up. And suddenly, underneath your arm, your fingers detect a painful, tender lump – one that wasn't there yesterday. Your mind starts to race: "Do I need to worry about this? Could it be an infected lymph node, even though I haven't felt sick? Could it be… cancer?"

What is the lymphatic system?

Your body's lymphatic system, made up of a series of small vessels, carries a clear liquid – lymph – from your body tissues to the heart. In the heart, lymph joins blood and is pumped via arteries back to the tissues. This efficient system helps drain excess liquid from tissues, and transports infection-fighting white blood cells to where they're needed.

What are lymph nodes?

Scattered along these small lymphatic vessels are up to 700 lymph nodes. These small (think bean-sized) bundles of tissue produce and store lymphocytes: immune cells whose job it is to trap and kill foreign substances, including harmful bacteria and viruses.

Why do lymph nodes become swollen?

When your body detects a foreign substance, it sends white blood cells to destroy that substance. White blood cells are manufactured in the lymph nodes, and sometimes when your body is dealing with an infection, these nodes manufacture so many cells that they become swollen, and you can feel them. They also become sore and painful. Most often, you'll feel painful, swollen lymph nodes in your neck, groin, or under your arm.

So an underarm lump is a swollen lymph node?

Probably – but not always. And that's why it's important to take underarm lumps seriously, and not dismiss them as simply a side effect of being ill.

An underarm lump that appears suddenly and feels rather large (think marble-sized), especially if you've been feeling ill, is most likely to be a swollen lymph node. This lump will usually be soft and moveable, and feel sore or painful. You may even notice some reddening of the skin around the lump.

Unlike a swollen lymph node, which appears rather suddenly, a breast cancer lump grows much more slowly. At first you think you feel something, then maybe not. A few weeks (or months) later, you notice it again; it seems larger. It feels hard, seems fixed in place, and may be a bit sore, but really not too much.

Should I see a doctor right away?

If the lump you feel in your armpit is sore, perhaps red, and has appeared suddenly – usually overnight – it's probably a swollen lymph node. It's safe to wait awhile, to see if the pain and swelling go down on their own. A swollen lymph node, while it may remain enlarged for several weeks, usually becomes less painful within several days.

If the lump you feel isn't sore, or barely sore; and it lingers for several weeks, then it's wise to see a doctor, in order to eliminate the possibility of breast cancer.

How can breast cancer develop in your armpit?

Your breast tissue actually extends beyond the boundaries of what you'd consider to be your breasts. Your underarms include breast tissue, and thus a cancerous tumor can develop in breast tissue under your arm – just as it would in your actual breasts.

Underarm lumps can also be cysts; or fibroadenomas, harmless tumors that result from hormonal fluctuations. So invasive cancer and swollen lymph nodes aren't the only causes of underarm lumps.

But it's important not to dismiss an underarm lump as "probably just a swollen lymph node" or "nothing important." Because not taking that underarm lump seriously – making an assessment of its characteristics, and determining whether and when to see a doctor – could potentially be fatal.

See more helpful articles:

A Guide to Breast Cancer Symptoms

Fast FAQS: Armpit Lumps

Breast Lump FAQS

Top 10 Breast Cancer Risk Factors

Breast cancer survivor and award-winning author PJ Hamel, a long-time contributor to the HealthCentral community, counsels women with breast cancer through the volunteer program at her local hospital. She founded and manages a large and active online survivor support network.