The body’s immune system relies on a vast network of vessels and small, bean-shaped organs known as lymph nodes. The axillary lymph nodes, located in the armpit region, are often a source of concern because they are easily palpable when enlarged. Understanding the typical size and features of these nodes helps distinguish between a harmless immune reaction and a potential health concern. This article provides clarity on normal axillary lymph node size and when a medical evaluation is warranted.
The Function and Location of Axillary Lymph Nodes
The axillary region, or armpit, is home to a cluster of 20 to 40 lymph nodes. These nodes are positioned in the deep tissues and fat of the underarm, divided into groups such as the pectoral, lateral, and central nodes. Their location makes them the primary drainage site for lymph fluid originating from the upper limbs, the adjacent chest wall, and a significant portion of the breast tissue.
The primary function of lymph nodes is to filter lymph fluid before it returns to the bloodstream. This filtration traps foreign particles, cellular waste, and microorganisms, preventing them from circulating throughout the body. The nodes are densely packed with specialized immune cells, including lymphocytes and macrophages, which neutralize pathogens or abnormal cells. When the body detects a threat, these immune cells rapidly multiply, causing the nodes to swell.
Defining Normal Axillary Lymph Node Size
In a healthy state, an axillary lymph node is small and often not noticeable to the touch. On imaging, such as an ultrasound, a normal node is characterized by a specific shape and size. The established benchmark for a healthy axillary lymph node is a measurement of less than 10 millimeters (1 centimeter) along its shortest axis.
Beyond size, the internal structure is a defining feature of a normal node. A healthy node maintains an oval or kidney-bean shape with smooth, well-defined margins. A distinguishing characteristic is the presence of a central fatty hilum, which appears bright on ultrasound and indicates normal internal organization. The outer layer, or cortex, should be uniformly thin, measuring 3 millimeters or less.
When assessed by hand, normal axillary nodes are often non-palpable, though small, soft nodes may be felt. These nodes are mobile, meaning they can be gently moved beneath the skin, and they should not be tender or painful. A short-to-long axis ratio, where the node is significantly longer than it is wide, also supports a benign, normal morphology.
Causes of Temporary Lymph Node Enlargement
The most frequent reason an axillary lymph node temporarily exceeds normal size is reactive lymphadenopathy, signifying an active immune response. This benign enlargement results from the node proliferating immune cells to combat infection or inflammation in the drainage area. Common triggers include local infections, such as a cut, insect bite, or skin infection on the arm, hand, or chest wall.
Recent vaccinations are another well-documented cause of temporary axillary node swelling. Both the flu shot and the COVID-19 vaccines often stimulate the immune system in the arm where the injection was given, causing the nodes in the corresponding armpit to enlarge. This reaction is usually unilateral, meaning it only occurs on the side of the injection, and the nodes may be slightly tender.
In these temporary, reactive cases, the swelling is expected to resolve as the body clears the underlying cause. While the nodes can swell to the size of a grape or larger, they usually soften and reduce in size within a few weeks (typically within two weeks). Systemic viral illnesses, like mononucleosis or certain respiratory infections, can also cause widespread or bilateral lymph node swelling in the armpits and other areas.
Size and Features Requiring Medical Evaluation
While most instances of axillary node enlargement are benign, certain features suggest the need for medical evaluation. Size alone is a factor, with persistent lymph nodes measuring greater than 1.5 to 2 centimeters often raising suspicion. However, the shape and texture of the node are often more significant indicators than size alone.
Nodes that are firm, rubbery, or hard to the touch, rather than soft, are considered concerning features. Similarly, a node that lacks mobility and feels fixed or matted to the surrounding skin or deep tissue requires immediate attention. On imaging, a node that loses its oval shape and becomes rounded is a significant morphological change associated with potential malignancy.
Loss or effacement of the central fatty hilum and an increased thickness of the outer cortex, especially if it exceeds 3 millimeters, may suggest cancer spread or lymphoma. Persistence is a strong signal for evaluation; any enlarged node that fails to shrink after four to six weeks, or one that continues to grow, should be promptly assessed by a healthcare provider to rule out underlying systemic or malignant conditions.