Axillary lymphadenopathy refers to enlarged lymph nodes in the armpit, and its causes range from ordinary immune responses to serious diseases like breast cancer and lymphoma. Most people notice it as a tender or painless lump under the arm, sometimes after an infection or vaccination, sometimes seemingly out of nowhere. Because the armpit contains roughly 20 to 40 lymph nodes that filter fluid from the arm, chest wall, and breast, swelling in this area can reflect problems originating from any of those regions, which is why doctors take it seriously even when the most likely explanation turns out to be benign.
Why the Armpit Is a Diagnostic Crossroads
The axilla is one of the body’s densest clusters of lymph nodes, and it acts as a drainage hub for the upper limb, the breast, and parts of the chest wall and upper back. Axillary masses can arise from any tissue component present in the region, including breast tissue that occasionally extends into the armpit. Beyond lymph node enlargement itself, the differential diagnosis includes soft tissue tumors like lipomas and nerve sheath tumors, post-surgical fluid collections, and even lesions in accessory breast tissue.1PubMed. Review of axillary lesions, emphasising some distinctive imaging and pathology findings When someone feels a lump under the arm, the first clinical task is figuring out whether it is actually a swollen lymph node or something else entirely.
Assuming the lump is indeed a lymph node, the next question is why it grew. Lymph nodes swell when they are mounting an immune response, trapping foreign material, or being infiltrated by abnormal cells. Most acute swelling after a local skin infection or a recent vaccination settles on its own. Persistent or painless enlargement, particularly nodes that feel hard or fixed to surrounding tissue, raises more concern and usually prompts imaging or biopsy.
Infections That Target Axillary Nodes
The most common infectious cause of armpit lymph node swelling is straightforward: a bacterial skin infection, wound, or insect bite on the hand, arm, or chest drains into the axillary nodes, which temporarily enlarge as part of the immune response. These reactive nodes are usually tender, somewhat soft, and shrink once the infection clears.
Cat-scratch disease is a less obvious culprit that clinicians encounter regularly enough to keep on the radar. It is caused by the bacterium Bartonella henselae, transmitted through a scratch or bite from an infected cat. The resulting lymphadenopathy can be dramatic, with nodes growing large enough over weeks to months that the initial suspicion is lymphoma. One case report described a young man with painful, progressively enlarging masses in his upper arm and armpit over four months; his initial workup pointed toward lymphoma until his history of sleeping with and being bitten by his cat steered clinicians toward biopsy, which confirmed cat-scratch disease.2PubMed Central. Cat-Scratch Lymphadenitis Presenting with Right Upper Arm and Axillary Masses: a Case Report Mimicking Lymphoma and Potential Diagnostic Pitfall In another case, an older woman developed a painless axillary mass that only became symptomatic months later with fever and chills. Standard antibiotics that do not cover Bartonella proved useless until a lymph node biopsy revealed the true diagnosis, and she improved rapidly on targeted therapy.3PubMed Central. Axillary cat-scratch disease: A case report and literature review The takeaway with cat-scratch disease is that it mimics cancer convincingly on physical exam and imaging, and a history of cat exposure is sometimes the strongest early clue.
Tuberculosis and related mycobacterial infections can also show up in axillary nodes, though this is far less common in higher-income countries. On ultrasound, mycobacterial nodes tend to appear round and dark, lacking the normal bright center (the hilum) that healthy nodes display, with increased blood flow in a branching pattern.4Radiology of Infectious Diseases. Tuberculous lymphadenitis in the left axillary misdiagnosed as metastasis These features can look virtually identical to cancer spread on imaging, which is why at least one reported case of tuberculous axillary lymphadenitis was initially misdiagnosed as metastatic disease. TB and breast cancer can even coexist in the same node, complicating the picture further.5PubMed. Coexistence of breast cancer and tuberculosis in axillary lymph nodes: a case report and literature review Non-tuberculous mycobacteria, typically seen in people with weakened immune systems, have also been reported as a rare cause of axillary lymphadenitis in otherwise healthy individuals.6Respiratory Medicine Case Reports. A case of axillary lymphadenitis caused by Mycobacterium intracellulare in an immunocompetent patient
When Cancer Is the Cause
The fear that axillary lymphadenopathy represents cancer is understandable, because the axilla is one of the first places breast cancer spreads. Axillary lymph node status remains a central factor in staging breast cancer and guiding treatment decisions. Melanoma on the arm or upper trunk also drains to axillary nodes and can produce metastatic disease there.7PubMed. Adjuvant irradiation for axillary metastases from malignant melanoma Lymphomas, both Hodgkin and non-Hodgkin, frequently involve axillary nodes as well.
An interesting nuance in breast cancer biology is that finding cancer cells in axillary lymph nodes does not necessarily mean those nodes are the launching pad for spread to distant organs. Genomic research has found that axillary lymph node metastases tend to be genetically diverse compared with metastases found in distant organs like the lungs or liver. This suggests the axillary nodes are not the crucial waystation for distant spread that older models assumed; instead, their involvement reflects the cancer’s overall capacity to travel, rather than serving as the literal origin of life-threatening metastases.8JCI Insight. Evolutionary history of metastatic breast cancer reveals minimal seeding from axillary lymph nodes This shift in understanding has influenced how aggressively surgeons approach the axilla, a point that matters a great deal for patient quality of life after surgery.
Autoimmune and Inflammatory Causes
Swollen lymph nodes are a well-recognized feature of autoimmune diseases, and the axilla is a favored location for certain conditions. In one study of patients with active rheumatoid arthritis, roughly four out of five had enlarged lymph nodes, and those nodes were most commonly found in the axillary region. Patients with lupus also showed lymphadenopathy, though their nodes tended to be smaller and more widely distributed across the body rather than clustering in the armpit.9PubMed. Frequency of lymphadenopathy in rheumatoid arthritis and systemic lupus erythematosus For rheumatoid arthritis, axillary lymph nodes are sometimes visible on routine chest CT scans, and earlier reports treated this as a systemic manifestation of the disease rather than simply local drainage from inflamed joints in the hands and wrists.10PubMed. CT assessment of axillary lymphadenopathy in patients with rheumatoid arthritis: association with disease activity and severity
Less common inflammatory conditions can also target axillary nodes. Kikuchi-Fujimoto disease, a self-limiting condition that usually affects cervical (neck) lymph nodes in young women, occasionally presents in the axilla. Published estimates put the rate of axillary involvement at anywhere from a few percent to as high as 40 percent of cases, though the wide range reflects the rarity of the disease and the small sample sizes available.11CytoJournal. Fine needle aspiration diagnosis of Kikuchi’s lymphadenitis in axillary lymphadenopathy Sarcoidosis, another granulomatous condition, can involve axillary nodes too, though it more classically targets the hilar nodes deep in the chest.
Post-Vaccination Lymphadenopathy
COVID-19 vaccination brought axillary lymphadenopathy into mainstream awareness. Swollen nodes on the side of the injection arm became one of the most-discussed side effects, partly because it triggered anxiety about breast cancer and partly because it showed up on screening mammograms and created diagnostic confusion. This was not a new phenomenon—any intramuscular vaccine delivered to the deltoid can cause reactive armpit nodes—but the massive scale of COVID-19 vaccination made it newly visible.
Prospective ultrasound studies tracked how long the swelling persists. In one study, about a quarter of women had complete resolution by six weeks after vaccination, but roughly half still showed persistent lymphadenopathy at three months.12PubMed Central. US Evaluation of Axillary Lymphadenopathy Following COVID-19 Vaccination: A Prospective Longitudinal Study Another study looking specifically at booster doses found that lymphadenopathy resolved on average about 100 days after the booster, somewhat faster than after the first dose in the primary series, which took about 130 days. Age, vaccine brand, and a history of breast cancer did not significantly affect how long the nodes stayed enlarged.13PubMed. Axillary Lymphadenopathy After a COVID-19 Vaccine Booster Dose: Time to Resolution on Ultrasound Follow-Up and Associated Factors
Vaccine type did matter, though. Comparisons between mRNA vaccines and vector-based vaccines found that mRNA recipients had more visible nodes, thicker cortices, and more blood flow on Doppler imaging. Younger patients also showed more pronounced changes. By three months, the findings had normalized in more than 80 percent of patients regardless of vaccine type.14PubMed Central. Axillary lymph node imaging in mRNA, vector-based, and mix-and-match COVID-19 vaccine recipients: ultrasound features For anyone who notices a lump under the arm after a vaccination, the practical message is that it is very common, almost always harmless, and usually fades within a few months. However, if it persists beyond 12 weeks or has features that look suspicious on imaging, follow-up is still warranted to rule out other causes.
Silicone Implants and Tattoo Pigment
Two iatrogenic causes of axillary lymphadenopathy catch both patients and clinicians off guard. The first involves silicone breast implants. When an implant ruptures, or even through gradual “silicone bleeding” from an intact shell, silicone particles migrate into surrounding tissue and are carried by immune cells to nearby lymph nodes. The resulting foreign body reaction causes chronic inflammation and swelling that can look alarmingly similar to metastatic cancer on imaging.15PubMed Central. Extensive silicone lymphadenopathy after breast implant insertion mimicking malignant lymphadenopathy Silicone lymphadenopathy does not stay confined to the axilla, either. Silicone transported by macrophages can travel to supraclavicular nodes above the collarbone and potentially beyond, compounding the diagnostic challenge.16PubMed Central. Supraclavicular lymphadenopathy: an unexpected consequence of breast implant rupture For anyone with breast implants who discovers a swollen lymph node, the possibility of silicone migration should be raised with the treating physician before assuming malignancy.
The second surprise is tattoo pigment. Ink particles deposited in the skin can slowly migrate through lymphatic vessels to regional lymph nodes over years or decades. If the tattoo is on the arm, shoulder, or upper trunk, those particles end up in axillary nodes. One reported case described axillary lymphadenopathy appearing 30 years after a decorative tattoo, closely mimicking metastatic melanoma on clinical examination.17PubMed Central. Tattoo pigment in an axillary lymph node simulating metastatic malignant melanoma In sentinel lymph node biopsies performed for actual melanoma, tattoo pigment in the node can be mistaken for melanoma deposits, potentially leading to unnecessary further surgery.18PubMed. Pigmentation of axillary sentinel nodes from extensive skin tattoo mimics metastatic melanoma Pathologists experienced with tattoo pigment can usually tell the difference under the microscope, but the initial alarm on gross examination can be significant.
How Axillary Lymphadenopathy Is Evaluated
The clinical evaluation typically starts with a physical exam and history. Doctors assess the size, texture, and tenderness of the node, and ask about recent infections, vaccinations, animal exposures, and cancer history. Physical examination alone has limited accuracy for distinguishing benign from malignant nodes. In the context of breast cancer staging, clinical palpation has been reported to have a sensitivity of around 30 percent and a specificity of around 93 percent, meaning it misses many involved nodes but is fairly reliable when it does detect something abnormal.19PubMed Central. Lymph Node Imaging in Patients with Primary Breast Cancer: Concurrent Diagnostic Tools
Ultrasound is typically the first imaging step for axillary lymphadenopathy because it can evaluate characteristics that CT and MRI largely cannot, such as the shape of the node, whether it has a preserved fatty hilum, the thickness and regularity of the cortex, and the pattern of blood flow within it. Early signs of cancer involvement include asymmetric cortical thickening and focal bulging, while later changes include complete loss of the hilum and diffuse thickening.20PubMed. Ultrasound of superficial lymph nodes When ultrasound uses these shape-based criteria rather than relying purely on node size, specificity for detecting metastatic disease reaches roughly 88 to 98 percent.21PubMed Central. Lymph Node Imaging in Patients with Primary Breast Cancer: Concurrent Diagnostic Tools
If a node looks suspicious on ultrasound, the next step is usually a needle biopsy performed under ultrasound guidance. Core needle biopsy provides a tissue sample large enough for a definitive diagnosis and has a higher diagnostic yield than fine-needle aspiration. The procedure is generally safe in experienced hands, given proper understanding of the surrounding anatomy and the use of a biopsy device with controllable needle action.22Radiographics. US-guided core needle biopsy of axillary lymph nodes in patients with breast cancer: why and how to do it This pathway—ultrasound first, then targeted biopsy if warranted—avoids unnecessary surgical procedures and gives clinicians a tissue diagnosis before planning any definitive treatment.
What Happens When Axillary Nodes Are Surgically Removed
When cancer is confirmed in axillary nodes, treatment historically involved removing most of them through a procedure called axillary lymph node dissection. The morbidity of this surgery is significant. In the well-known Z0011 trial, about 70 percent of patients who underwent both sentinel node removal and full axillary dissection reported adverse surgical effects, compared with 25 percent who had sentinel node surgery alone. Wound infections, fluid collections (seromas), and numbness or tingling (paresthesias) were all considerably more common with the more extensive procedure. At one year, about 13 percent of patients who had full dissection reported lymphedema, the chronic arm swelling that can follow disruption of lymphatic drainage, compared with 2 percent after sentinel node surgery alone.23PubMed. Surgical complications associated with sentinel lymph node dissection (SLND) plus axillary lymph node dissection compared with SLND alone in the American College of Surgeons Oncology Group Trial Z0011
These findings, along with the genomic evidence that axillary node metastases may not be the primary route for distant cancer spread, have pushed oncology toward less aggressive axillary surgery in many situations. For patients with limited nodal disease, sentinel node biopsy alone is increasingly considered sufficient. When full dissection is still required, it can sometimes be performed as an outpatient procedure, though seroma formation remains the most common complication, occurring in roughly a third of patients in one series. Most of those seromas required only a single drainage and were manageable without hospitalization.24PubMed. Axillary node dissection in outpatient procedure, is it feasible and safe? Newer classification systems for axillary lymph node anatomy aim to help surgeons target their dissection more precisely, reducing the risk of lymphedema by preserving nodes and lymphatic channels that do not need to be removed.25PubMed Central. New classifications of axillary lymph nodes and their anatomical-clinical correlations in breast surgery
When a Swollen Armpit Node Deserves Prompt Attention
Not every swollen lymph node in the armpit needs a workup. Small, soft, mobile nodes that appear during or shortly after an infection on the arm or hand, or in the weeks following a vaccination, are almost always reactive and will resolve. The features that prompt clinicians to dig deeper include nodes that persist beyond four to six weeks without an obvious cause, nodes that are hard or fixed to surrounding tissue, nodes that keep growing, and any painless lump in someone with a personal history of cancer.
Age matters in the calculus. In children, axillary lymphadenopathy is overwhelmingly reactive, usually driven by minor infections. In adults over 40, the probability that an unexplained persistent node is malignant rises enough that imaging and biopsy are pursued more readily. Accompanying symptoms like unexplained weight loss, drenching night sweats, or persistent fevers raise the urgency further, since these are associated with lymphoma and other systemic malignancies.
For people with breast implants, the differential should always include silicone lymphadenopathy. For cat owners, especially those with kittens (which carry Bartonella henselae at higher rates), cat-scratch disease belongs on the list. For anyone with extensive tattoos on the arm or upper body, pigment migration is a plausible explanation for a node that looks dark or discolored at biopsy. Knowing these less obvious causes helps patients advocate for appropriate testing rather than jumping straight to the assumption that any axillary lump means cancer.