Allergies, vaccination reactions, and bacterial, fungal or viral infections can all cause lumps underneath the arm, according to Healthline. Sometimes harmless growths composed of fatty tissue, called lipomas, or benign fibrous tissue growths, called fibroadenomas, cause lumps in the armpit. However, serious conditions such as lymphoma, breast cancer, leukemia, lupus and systemic lupus erythematosus can also cause armpit lumps.
Because armpit lumps are a possible sign of the body's response to breast cancer, women should report any bumps in this area to their doctor, explains Healthline. Women should also conduct a monthly breast exam one to three days after the end of their period to check for other breast lumps.
To determine the cause of an armpit lump, a doctor conducts a physical examination by palpating the area, states Healthline. The doctor also asks the patient about any pain and any changes associated with the lump. If necessary, the doctor may order a biopsy, mammogram, blood count or allergy testing to confirm or reach an accurate diagnosis.
Treatment for an armpit lump varies depending on the underlying cause, reports Healthline. Cancerous lumps may require surgery, chemotherapy or radiation. Doctors treat lumps caused by a bacterial infection with antibiotics. Although it is not medically necessary, a surgeon can remove lumps that are lipomas or fibroadenomas if they are large enough to bother the patient. Other lumps may disappear as the infection or allergic reaction dissipates.
Once diagnosed, patients can treat uncomfortable lumps at home with over-the-counter medication and warm compresses, suggests Healthline.