The discovery of a lump under your armpit (axilla) can be alarming. This region contains a dense network of lymph nodes, blood vessels, and fatty tissue, making lumps common. While serious illness is a concern, most armpit lumps are non-serious, resolve without intervention, and originate from immune responses or localized skin issues.
Lumps Caused by Immune System Activity
The most frequent cause of an armpit lump is the temporary enlargement of a lymph node, a process called reactive lymphadenopathy. Lymph nodes are small, bean-shaped structures in the lymphatic system that filter the body, trapping and destroying invaders like bacteria and viruses using specialized white blood cells.
When the body detects an infection or inflammation in an area the armpit nodes drain—such as the arm, hand, or breast—the nodes swell as they ramp up production of infection-fighting cells. This swelling, which can make the node feel firm and tender, is a sign that the immune system is actively working. Common causes for this reaction include a minor cold, flu, a localized skin infection, or even a reaction to a recent vaccination.
These reactive nodes are typically movable, tender to the touch, and often return to their normal size within a few weeks as the underlying issue resolves. The swelling can range from the size of a pea to that of a golf ball, depending on the intensity of the immune response.
Benign Skin and Tissue Growths
Lumps in the axilla can also originate from the local skin and soft tissue, independent of a systemic immune response. A lipoma is a common type of non-cancerous lump, consisting of a slow-growing collection of fatty tissue. These lumps are usually soft, doughy to the touch, and easily movable beneath the skin, rarely causing pain.
Another frequent occurrence is an epidermal cyst, which forms when surface skin cells move deeper into the skin and multiply, creating a sac filled with a cheesy substance called keratin. These cysts feel smooth and are generally fixed to the skin’s surface, sometimes developing a small, dark plug. Inflammation of a hair follicle, known as folliculitis, or a deeper pocket of infection called an abscess, can also create a tender, red, and warm lump.
A more chronic skin condition that causes lumps is Hidradenitis Suppurativa (HS), which involves the blockage and inflammation of hair follicles, often in the armpit. HS causes deep-seated, painful nodules that can rupture and leak pus, sometimes forming tunnels called sinus tracts under the skin. Unlike a simple cyst, HS lesions tend to recur and can lead to scarring over time in the affected area.
When Armpit Lumps Indicate Serious Disease
While most lumps are benign, an armpit mass can occasionally signal a more serious underlying health issue, particularly a malignancy. The primary concern is often the spread of breast cancer, as the axillary lymph nodes are the first place cancer cells travel from the breast via the lymphatic system. When cancer has spread, the lymph nodes become enlarged due to the presence of malignant cells, a process known as metastasis.
These cancerous nodes are often described as feeling hard, fixed, or immovable within the tissue, and they are typically painless. The lump may also be a manifestation of a primary cancer of the immune system, such as lymphoma or leukemia.
In lymphoma, the cancer originates in the lymphocytes within the lymph nodes, causing them to swell and become firm. These malignant lumps, whether from metastasis or a primary lymphoma, tend to persist for longer than a few weeks and may be accompanied by systemic symptoms. A persistent, enlarging, or fixed lump warrants immediate medical evaluation to determine its exact nature.
How Doctors Diagnose the Cause
Diagnosis begins with a thorough physical examination and a detailed review of the patient’s medical history. The doctor carefully feels the lump, noting its size, texture, mobility, and tenderness, while checking for signs of infection or inflammation. Questions about recent infections, vaccinations, trauma, and associated systemic symptoms are critical initial steps.
If the lump is concerning or the cause is not immediately clear, imaging studies are often ordered. Ultrasound is the most common first-line tool, using sound waves to create a real-time image and determine if the lump is solid tissue or fluid-filled. For women, a mammogram may be performed simultaneously to evaluate the breast tissue for any underlying issues that could be the source of a swollen lymph node.
The definitive procedure for diagnosing a suspicious solid mass is a biopsy, which involves taking a small tissue sample for laboratory analysis. This may be a fine-needle aspiration (FNA), where a thin needle is used to collect cells, or a core needle biopsy, which extracts a larger cylinder of tissue. The pathologist’s analysis of the biopsied cells provides the most accurate diagnosis, distinguishing between benign, inflammatory, and malignant causes.
Knowing When to Seek Immediate Medical Attention
While many armpit lumps resolve on their own, certain characteristics serve as “red flags” that require prompt medical consultation. Any lump that is rapidly increasing in size or that persists for more than two to four weeks without any signs of improvement should be evaluated by a healthcare professional. Lumps that feel extremely hard or are fixed and immovable to the underlying tissue are particularly concerning and demand immediate attention.
Accompanying changes in the skin over the lump, such as dimpling, puckering, or unusual discoloration, also signal the need for a timely doctor’s visit. Beyond the physical characteristics of the lump itself, systemic symptoms occurring at the same time are also important indicators. These signs include an unexplained fever, drenching night sweats, or unintentional weight loss.
If the lump is painful, red, and warm, it may indicate a localized infection requiring antibiotic treatment and medical assessment. A painless lump that is hard and fixed can be more concerning than a painful one, as pain often suggests an inflammatory process. Consulting a healthcare provider about any unusual or persistent change ensures the most accurate diagnosis and appropriate management.