A lump in the armpit may appear as a bump or swelling under the arm and can have many different causes.
The most frequent cause of a lump in the armpit is an enlarged lymph node, which is a part of the immune system that may swell if the body is fighting an infection.
See a doctor if you have a lump in your armpit and the lump:
- Is accompanied by redness, pain, or fever
- Is very firm
- Does not go away
- Gets bigger
- You are female (may be a sign of breast cancer)
While a lump in the armpit can be a sign of cancer, it is not possible to diagnose cancer by touch alone. If you have a lump in your armpit and are at all concerned, talk to your doctor.
What Causes a Lump in My Armpit?
A lump in the armpit can have many causes, such as:
- Enlarged lymph nodes
- Arm or breast infection
- Systemic infections, such as mononucleosis (“mono”), acquired immunodeficiency syndrome (AIDS), herpes, or other bacterial or viral infections
- Cancers, such as breast cancer, leukemias, or lymphomas
- Cysts or abscesses under the skin
- Shaving
- Use of antiperspirants (not deodorants)
- Allergic reactions
- Cat scratch disease
- Benign fatty growths (lipomas)
- Fibroadenomas
- Use of certain medicines
- Reaction to vaccinations
- Hidradenitis suppurativa
- Systemic lupus erythematosus (SLE)
How Is a Lump in My Armpit Diagnosed?
The cause of a lump in the armpit is diagnosed with a patient history and physical examination of the area, which involves the doctor feeling the lump. Depending on the suspected cause of the lump, tests may include:
- Blood tests
- Allergy testing
- Mammogram
- Computerized tomography (CT) scan
- Magnetic resonance imaging (MRI) scan
- Fluid culture from the lump to check for infection
- Tissue biopsy
What Is the Treatment for a Lump in My Armpit?
Treatment for a lump in the armpit depends on the cause. Often, lumps are benign and don’t need treatment.
Treatment for a lump in the armpit due to infection may include antibiotics.
Treatment for a lump in the armpit due to allergies may include:
- Avoiding allergens
- Antihistamines
- Allergy shots
Treatment for a lump in the armpit due to hidradenitis suppurativa may include:
Treatment for a lump in the armpit due to cancer will depend on the type of cancer and the stage and may include:
- Chemotherapy
- Radiation therapy
- Immunotherapy
- Targeted therapy
- Surgery
From 
Image source: iStock Images
https://www.mountsinai.org/health-library/symptoms/armpit-lump
https://blog.dana-farber.org/insight/2020/07/should-i-worry-about-a-lump-under-the-armpit/
https://www.aad.org/public/diseases/a-z/hidradenitis-suppurativa-treatment
https://www.cancer.org/cancer/breast-cancer.html