The human body contains hundreds of small, bean-shaped structures known as lymph nodes, which function as decentralized filtering stations for the immune system. The axillary lymph nodes are a cluster of approximately 20 to 40 of these nodes located deep within the armpit, or axilla. Their primary role is to filter lymph fluid drained from the upper limbs, the chest wall, and a large portion of the breast tissue, trapping foreign invaders like bacteria and viruses. Swelling in these nodes, medically termed lymphadenopathy, is a common physical sign that the immune system is actively working to contain a threat or clear cellular debris.
How Hormones Affect Axillary Lymph Nodes
Hormones do not directly cause axillary lymph nodes to swell like an infection does, but they can influence the surrounding tissue. The breast tissue extends laterally into the armpit, forming the axillary tail of Spence. This tissue is highly responsive to fluctuations in reproductive hormones like estrogen and progesterone.
During the high-hormone phase of the menstrual cycle, breast tissue often experiences temporary engorgement and fluid retention. This cyclical swelling extends into the armpit, causing the area to feel fuller, more sensitive, and sometimes painful. Although the lymph nodes themselves may not be pathologically enlarged, the surrounding swollen breast tissue can compress them, making normal-sized nodes feel distinctly enlarged or tender.
This hormonally influenced swelling is temporary and usually resolves completely once menstruation begins and hormone levels decline. It is typically bilateral, meaning both armpits are similarly affected, which helps distinguish it from localized causes of swelling. Systemic hormonal dysregulation, such as thyroid disorders, rarely causes isolated axillary swelling.
The Most Common Causes of Swollen Armpit Nodes
The most frequent causes of true lymphadenopathy are directly related to the immune system’s response to a localized threat. Infections are the predominant trigger, as the axillary nodes drain the entire arm and hand. Minor skin infections, such as those resulting from an ingrown hair, a cut from shaving, or a bug bite on the arm, can cause rapid, temporary swelling in the corresponding armpit.
Viral illnesses, including the common cold, influenza, or infectious mononucleosis, frequently cause lymph node enlargement throughout the body, including the axilla. This swelling indicates the body is mobilizing immune cells to fight the infection. Cat scratch disease, a bacterial infection transmitted by an infected feline, is another common cause of localized axillary lymphadenopathy, especially in children.
A common, temporary cause of axillary swelling is a recent vaccination, particularly those administered in the upper arm, such as the COVID-19 or flu shots. The vaccine stimulates a robust immune response, causing the lymph nodes on the injection side to become enlarged as they process the antigen. This reaction is a normal sign of immune system activation, and the swelling typically resolves within a few weeks. When swelling is caused by infection or irritation, it is usually unilateral.
Signs That Require Immediate Medical Evaluation
Although most instances of axillary lymph node swelling are benign, specific characteristics warrant prompt evaluation by a healthcare provider. The duration of the swelling is important; nodes that remain enlarged for longer than two to four weeks, even after an infection or irritation has cleared, should be examined. Persistent swelling suggests an ongoing process or a non-infectious cause.
The physical characteristics of the node offer important clues for diagnosis. Nodes that feel hard, fixed, or rubbery are more concerning than those that are soft and freely movable under the skin. While pain and tenderness are common with infection, a painless, firm, and growing lump without any obvious local cause requires medical attention.
Lymphadenopathy accompanied by systemic symptoms should not be ignored. These include unexplained weight loss of more than ten percent of body weight, a persistent fever that cannot be attributed to a common illness, or drenching night sweats. These signs, known as “B symptoms,” are sometimes associated with certain underlying conditions, making a medical assessment necessary to determine the cause.