To understand what an armpit lump might be, it’s helpful to consider the structures present in the axilla:

1. Lymph Nodes: These small, bean-shaped glands are part of the immune system, filtering lymph fluid and housing immune cells. They are numerous in the armpit, draining the breast, arm, and upper back.

2. Fatty Tissue: The armpit contains a significant amount of adipose tissue.

3. Skin and Subcutaneous Tissue: Hair follicles, sweat glands, and sebaceous glands are abundant in this area.

4. Muscles, Nerves, and Blood Vessels: While less common to form palpable lumps, issues with these structures can sometimes present as such.

Let’s explore the three most common causes of armpit lumps:

1. Enlarged Lymph Nodes (Lymphadenopathy)

Lymph nodes are often the first responders to infection or inflammation in their drainage area. When they enlarge, it’s typically a sign that your immune system is actively fighting something off. This is known as lymphadenopathy.

Causes of Enlarged Lymph Nodes:

• Infections: This is the most frequent cause. Bacterial infections (e.g., cellulitis, cat scratch disease, folliculitis), viral infections (e.g., common cold, mononucleosis, HIV, shingles), and fungal infections can all lead to reactive lymphadenopathy. The nodes become swollen, tender, and sometimes painful as they fill with immune cells.
• Inflammatory Conditions: Autoimmune diseases like rheumatoid arthritis or lupus can cause generalised lymph node enlargement, including in the axilla.
• Allergic Reactions: Severe allergic reactions can sometimes trigger regional lymph node swelling.
• Vaccinations: Recent vaccinations (e.g., COVID-19, MMR) in the arm can cause temporary enlargement of axillary lymph nodes on the vaccinated side.
• Malignancy: This is the most concerning cause. Lymph nodes can enlarge if they are involved in metastatic cancer (e.g., breast cancer, melanoma, lung cancer) or primary cancers of the lymphatic system (lymphoma). Malignant nodes are often firm, non-tender, and fixed to surrounding tissues. They may also be accompanied by constitutional symptoms.

Clinical Presentation: Reactive lymph nodes are typically soft to firm, mobile, and often tender or painful to touch. They usually resolve as the underlying infection or inflammation subsides. Malignant nodes, conversely, tend to be harder, non-tender, and may feel fixed. The size can vary significantly, from a few millimetres to several centimetres.

2. Lipomas

A lipoma is a benign (non-cancerous) tumour composed of fatty tissue. They are among the most common soft tissue tumours and can occur anywhere on the body where fat cells are present, including the armpit.

Causes of Lipomas: The exact cause of lipomas is unknown, but they tend to run in families, suggesting a genetic predisposition. They are more common in middle-aged adults and can sometimes develop after a minor injury, though the link is not fully understood.

Clinical Presentation: Lipomas are typically:

• Soft and doughy to the touch: They feel like a rubbery lump under the skin.
• Mobile: They can be easily moved with finger pressure.
• Painless: Most lipomas are asymptomatic, though they can become painful if they grow large enough to press on nerves or if they are located in an area subject to friction.
• Slow-growing: They usually grow very slowly over months or years and rarely exceed a few centimetres in diameter.
• Located just beneath the skin: They are subcutaneous.

Lipomas are generally harmless and do not require treatment unless they are causing symptoms (pain, cosmetic concern) or if their diagnosis is uncertain.

3. Cysts

Cysts are sacs filled with fluid, semi-solid material, or air. In the armpit, common types include epidermal cysts (also known as sebaceous cysts) and hidradenitis suppurativa-related cysts.

Causes of Cysts:

• Epidermal Cysts: These arise from hair follicles or blocked sebaceous glands. They occur when skin cells, instead of shedding, move deeper into the skin and multiply, forming a sac that fills with keratin (a protein found in skin and hair). They are very common.
• Hidradenitis Suppurativa (HS): This is a chronic inflammatory skin condition affecting areas with apocrine sweat glands, such as the armpits, groin, and buttocks. It causes painful, deep-seated nodules, abscesses, and sinus tracts that can resemble cysts.

Clinical Presentation:

• Epidermal Cysts: Typically smooth, round, and mobile lumps under the skin. They can range in size from small to several centimetres. They are usually non-tender unless they become inflamed or infected, in which case they can become red, swollen, and painful, sometimes discharging a foul-smelling, cheesy material.
• Hidradenitis Suppurativa: Presents as recurrent, painful, inflamed lumps that can rupture and drain pus. Over time, HS can lead to scarring and the formation of interconnected tunnels (sinus tracts) under the skin. It is often associated with significant discomfort and can have a profound impact on quality of life.