Swollen lymph nodes under the arm: causes and treatment options

Axillary lymphadenopathy is enlargement and/or tenderness of the lymph nodes in the armpit. It is most often a reactive response to localized skin and soft tissue infections of the arm/shoulder girdle or after vaccination of the shoulder. Less commonly, specific infections (such as cat-scratch disease) and systemic diseases are the cause. In some adults, the axillary nodes are involved in tumor processes (primarily breast cancer), so the physician always evaluates the context and "red flags." [1]

It's important to distinguish reactive lymphadenopathy (enlargement without pus) from lymphadenitis (inflammation of the node with pain, redness, and sometimes suppuration). Reactive nodes are often soft, mobile, and moderately painful; lymphadenitis can cause severe pain, skin hyperemia, and fever. Proper management helps avoid unnecessary antibiotics and biopsies, while also identifying serious underlying causes early. [2]

Since 2021, doctors have frequently seen vaccine-associated axillary lymphadenopathy on the injection site (including COVID-19 and other vaccinations). This is a benign reaction that can last longer than initially expected—weeks or even months. Currently, it is not recommended to delay screening mammograms due to vaccination; simply notifying the radiologist is sufficient. [3]

At the same time, axillary nodes are a key "station" for breast tumors: in patients with neoplasms, the diagnostic and treatment strategy is carefully calibrated down to the last detail (ultrasound-guided biopsy, sentinel node biopsy, avoiding excessive surgery in cases of low risk). In everyday practice, this means one thing: if the nodes are persistently enlarged on one side without an obvious benign cause, an in-person evaluation is essential. [4]

Epidemiology

In the general population, axillary nodes most often enlarge with local skin infections, after vaccinations, and with cat scratch disease (Bartonella henselae), especially in children and adolescents; the disease is usually self-limited within 2-4 weeks. However, in immunocompromised individuals, the course can be severe and disseminated. [5]

Following vaccination (including mRNA vaccines), the incidence of unilateral axillary lymphadenopathy is noticeable in mammography programs. Studies show that the increase can appear within 1-5 weeks and persist for an average of 3-4 months, and sometimes up to ≈40-43 weeks; this is why professional societies have abandoned advice on delaying screening mammography. [6]

In breast cancer, axillary nodes remain a key prognostic factor, but global guidelines increasingly limit the scope of axillary surgery (wide dissection is not necessary for everyone), which has reduced surgical complications without compromising oncological control. For the patient, this means that suspicious nodes will require targeted imaging and biopsy, rather than major surgery "just in case." [7]

In the outpatient treatment of adults with unexplained peripheral lymphadenopathy, benign causes generally predominate; the risk of serious pathology is increased by age >40-50 years, "hard" and fixed nodes, enlargement longer than 4-6 weeks, systemic symptoms ("B-symptoms"), and supraclavicular localization. These same principles apply to the axillary region. [8]

Reasons

Localized skin and soft tissue infections of the arm/shoulder girdle, microtrauma, and scratches are the most common household sources of reactive axillary node enlargement. Classic "clues" include a wound on the hand/forearm, felon, folliculitis, bites, and a recent manicure. Bacterial lymphadenitis may cause pain, redness, and even abscess formation. [9]

Cat-scratch disease (Bartonella henselae) is a common cause of unilateral axillary lymphadenitis following a scratch/bite, especially in children; the nodes are painful, may suppurate, and often have a "primary lesion" on the skin. The disease is self-limited in most cases, but severe/disseminated cases in vulnerable patients require antibiotic therapy and, rarely, surgical drainage. [10]

Vaccine-associated lymphadenopathy following injections into the deltoid muscle (COVID-19, HPV, influenza, etc.) is a benign and common reaction. It may not appear immediately, but may appear weeks later and persist for months. It is important to document the side and date of vaccination in referrals and ultrasound/mammography reports. If node enlargement persists, a follow-up ultrasound is performed, and if there are any suspicious findings, a biopsy is performed. [11]

Oncological causes: breast cancer metastases, less commonly lymphomas and other tumors. Clinical features, imaging, and morphology are crucial here: nodes with suspicious ultrasound features are punctured/biopsied under ultrasound guidance; further steps are determined by specialized oncology recommendations (sentinel node biopsy, selective dissection limitation, etc.). [12]

Risk factors

For infectious lymphadenitis: microtrauma and inflammatory skin diseases of the hands, contact with kittens/cats without flea control, "dirty" cuts, immunosuppression. In children and adolescents, the risk of Bartonella etiology is higher; in immunocompromised individuals, disseminated disease is more common. [13]

For vaccine-associated hypersensitivity: a recent injection into the shoulder on the side of the lymphadenopathy (it is important to indicate this in the referral), booster doses. The risk lies not in the reaction itself, but in diagnostic errors in the interpretation of images if the vaccination history is not taken into account. [14]

For tumor causes: age, personal/family history of cancer, identified breast lump, abnormal nipple discharge, "hard" and fixed nodules, and absence of an obvious benign cause. These features raise the threshold of suspicion and expedite referral to an oncologist. [15]

In all groups, duration >4-6 weeks, nodule growth, “B symptoms” (fever, night sweats, weight loss), supraclavicular/multiple zones are “red flags” requiring expedited imaging and morphological verification. [16]

Pathogenesis

In reactive lymphadenopathy, antigenic stimulation triggers immune cell proliferation and dilation of the node sinuses—the node enlarges but retains its normal architecture (oval shape, visible "gates"/chyle). This is also reflected in ultrasound: a "benign" profile—an oval node with preserved chyle and predominantly portal vascularization. [17]

In bacterial lymphadenitis, inflammation can lead to capsular edema, skin hyperemia, and, in some patients, suppuration (abscess). Clinically, this is characterized by pain, redness, and fluctuation; ultrasound reveals fluid-filled/heterogeneous areas. In such cases, drainage may be required in addition to antibiotics. [18]

The vaccine generates a local immune response with a temporary enlargement of the draining nodes. Why is the reaction prolonged? Immune centers within the node continue to "work" for weeks and months; however, the node remains structurally reactive. Therefore, if the enlargement persists without any significant ultrasound signs, dynamic observation is acceptable. [19]

Tumor lesions are characterized by infiltration of the cortex by tumor cells, loss of architecture and hilus, cortical thickening, changes in vascular pattern (peripheral/chaotic vascularization), and increased nodule stiffness as determined by elastography—a set of features used to perform targeted biopsy. Modern methods (elastography, contrast-enhanced ultrasound, high-frequency CEUS) further enhance accuracy. [20]

Symptoms

Reactive axillary lymphadenopathy typically presents as a moderately painful, mobile "pea/bean" under the arm; it is often discovered "accidentally" during hygiene. There is often an obvious cause in the drainage area: a cut, folliculitis, whitlow, or a recent shoulder vaccination. General well-being is normal or slightly impaired. [21]

With lymphadenitis (inflammation of the node), pain is severe, the skin may be red and hot, fever and limited arm movement are possible; with suppuration, fluctuation. These conditions require in-person evaluation and possibly antibiotics/drainage. [22]

Cat scratch disease causes a unilateral, painful, enlarged nodule (including axillary) 1-2 weeks after the scratch/bite; a "primary lesion" is often visible on the skin. In most cases, spontaneous resolution occurs within 2-4 weeks, but a longer course is possible. [23]

Red flags: a hard/fixed nodule, significant growth, no obvious cause, an accompanying palpable mass in the breast, weight loss, night sweats, and persistent fever. If these are present, imaging and biopsy should be performed immediately. [24]

Forms and stages

Clinically, the following are distinguished: reactive lymphadenopathy (enlargement without signs of purulent inflammation), acute bacterial lymphadenitis (inflammation/pus), vaccine-associated lymphadenopathy, and tumor-associated lymphadenopathy. The treatment strategy depends on the form: from observation to antibacterial therapy or biopsy. [25]

By duration: acute (up to 2 weeks), subacute (2-6 weeks), and chronic (>6 weeks). Reactive/vaccine-associated nodules usually decrease in size within weeks to several months; “stuck” and growing nodules will require further investigation. [26]

According to ultrasound criteria: "benign profile" (oval shape, preserved chyle, thin uniform cortex, portal vascularization, low stiffness) and "suspicious profile" (round shape, thickened/asymmetric cortex, disappearance of chyle, peripheral vascularization, high stiffness, necrosis/cysts). This stratification helps to decide whether to observe or biopsy. [27]

In oncology patients, separate scenarios are distinguished (clinically/ultrasound-negative axilla, suspicious axilla, sentinel node tactics, indications for dissection) - they are governed by specialized guidelines. [28]

Complications and consequences

Bacterial lymphadenitis can lead to abscess formation and phlegmon, which requires drainage. Delayed treatment increases the risk of infection spreading to surrounding tissues. With Bartonella, up to 10% of nodes can become suppurated; sometimes invasive interventions are required. [29]

Vaccine-associated lymphadenopathy itself is not dangerous; the main risks are diagnostic errors (unnecessary biopsies, delayed screening). Current recommendations emphasize: mammography should not be delayed, and vaccination history should be taken into account when interpreting results. [30]

Tumor causes pose a risk of progression of the underlying disease; "wasted time" due to self-monitoring for warning signs worsens the prognosis. On the other hand, excessive axillary surgery for breast cancer increases complications (lymphedema, pain), so selective approaches are now actively used. [31]

Psychological sequelae (anxiety, 'cancer scare') are common; a clear monitoring/investigation plan reduces stress and the likelihood of unnecessary interventions. [32]

Diagnostics

1) Clinical stage. It is important to collect anamnesis: injuries/wounds/boils on the arm, contact with cats, date and side of vaccination, recent infections, presence of a lump in the mammary gland, medications. Examination: size, pain, mobility, skin, comparison of the right/left side, search for a lesion in the drainage area. If the picture is "calm" in an adult, observation for 2-4 weeks is acceptable. [33]

2) Laboratory testing as indicated. In case of fever/suspected infection - complete blood count, C-reactive protein; in case of suspected Bartonella - serology/PCR; in immunocompromised individuals - an extended infectious search. A biopsy "just in case" with a typical vaccine connection and a benign ultrasound profile is not rushed. [34]

3) Visualization. When in doubt, the method of choice is axillary ultrasound: the shape, cortex, chyle, and vascular pattern are assessed. Modern additions include elastography and contrast-enhanced ultrasound (CEUS): their combination improves the accuracy of distinguishing reactive from malignant nodules. If breast cancer is suspected, axillary ultrasound is a standard procedure. [35]

4) Morphology. Suspicious or persistent/progressive nodes are punctured/biopsied under ultrasound guidance (fine-needle aspiration for metastases; core biopsy if necessary to assess architecture/lymphoma). In the oncologic pathway, decisions about sentinel node biopsy and the extent of surgery are made according to current clinical guidelines. [36]

Table 1. When to observe and when to examine

Situation	Tactics
Small, soft, movable lump; obvious cause (scratch/boil, recent vaccination)	Observation for 2-4 weeks; treatment of the source; ultrasound if desired/in doubt
Pain, redness, fever, fluctuation	Ultrasound; antibiotics; in case of an abscess - drainage
No cause, nodule is hard/fixed, growth >4-6 weeks, "B-symptoms", palpable mass in the breast	Ultrasound of the armpit and mammary gland ± mammography/MRI; targeted biopsy
Vaccine-associated lymphadenopathy >3 months	Control ultrasound; if there are suspicious signs, biopsy
[37]

Differential diagnosis

Reactive lymphadenopathy vs. lymphadenitis. Reactive lymphadenopathy is characterized by moderate tenderness, normal skin, and a benign ultrasound profile; lymphadenitis is characterized by severe pain, redness, possible fluctuation, and fever; ultrasound reveals heterogeneity/fluid. [38]

Bartonella henselae vs. pyogenic lymphadenitis. Bartonella is typically associated with a cat scratch, is a "primary element," and has a subacute course, often leading to suppuration. Pyogenic (staphylococcal/streptococcal) lymphadenitis is often acute, with severe intoxication. Serology/PCR and clinical assessment help differentiate between the two. [39]

Vaccine-associated lymphadenopathy vs. tumor. The following factors favor vaccination: unilaterality on the injection site, onset weeks later, benign ultrasound profile, and tendency to regress. The following factors favor a tumor: lack of association with vaccination, "evil" ultrasound features (round shape, thickened cortex, loss of chyle, peripheral blood flow), and concomitant findings in the mammary gland—requiring biopsy. [40]

Other causes. Less commonly, axillary nodes enlarge with lymphoma, tuberculosis, sarcoidosis, etc.; systemic symptoms, multiple areas of lymphadenopathy, and specific laboratory/imaging findings help to identify these. [41]

Treatment

Reactive lymphadenopathy without any "red flags." No "special" therapy is required: observation for 2-4 weeks, hand care, treatment of the underlying lesion (dermatitis, whitlow), and pain relief as needed. Antibiotics are not indicated unless there are signs of bacterial inflammation of the node. [42]

Acute bacterial lymphadenitis. Empirical antibiotics covering staphylococci and streptococci of the skin flora, local cooling/rest, follow-up after 48-72 hours. For an abscess, puncture/incision and drainage under ultrasound guidance. The choice of drug and duration are based on local protocols (usually 5-7 days, longer in case of complications). [43]

Cat scratch disease. Most cases are self-limiting; azithromycin may be considered to accelerate node regression, especially in large and painful lymphadenitis. Suppuration requires drainage; dissemination/immunosuppression requires prolonged antibiotic therapy under the supervision of an infectious disease specialist. [44]

Vaccine-associated lymphadenopathy. Treatment is not required; it is recommended to record the fact and side of vaccination in the report. If lymphadenopathy persists for >3 months, a follow-up ultrasound is performed; if there are suspicious signs, a biopsy is performed. Screening mammography is not postponed. [45]

Suspected tumor. Ultrasound-guided fine-needle aspiration/CORE biopsy of the node; then follow the oncologic route (including sentinel node biopsy in early breast cancer and selective omission of wide dissection in low-risk cases). This approach reduces lymphedema and other complications without worsening oncologic outcomes. [46]

Diagnostic innovations. In complex cases, combining elastography and contrast-enhanced ultrasound (CEUS) is useful – meta-analyses from 2025 show increased accuracy in distinguishing benign from malignant nodules; high-frequency CEUS (H-CEUS) adds sensitivity for superficial nodules. These are diagnostic tools, not a replacement for biopsy. [47]

Table 2. What helps and what doesn't help

Situation	Effective	Ineffective/not indicated
Reactive node without inflammation	Observation and treatment of the skin lesion	"Prophylactic" antibiotics
Bacterial lymphadenitis	Antibiotics, ultrasound guidance; in case of abscess - drainage	Apply warming agents to treat pus
Bartonella	Azithromycin (accelerates regression), drainage in case of suppuration	Aggressive surgery without indications
Vaccine-associated node	Observation, control ultrasound at >3 months	Postponing mammograms/screening
Suspected tumor	Ultrasound-guided biopsy, oncomort	Long-term "self-observation" with "flags"
[48]

Prevention

Prevention of infectious causes: hand and armpit skin care, prompt treatment of wounds, proper hygiene during manicures/depilation, and hand protection during gardening. For cats, this includes flea control, avoiding play with kittens that can lead to scratches, and promptly treating fresh scratches. [49]

Preventing diagnostic errors: Always inform your doctor of the date/date of recent vaccinations and include this information in your ultrasound/mammogram referral; if any "red flags" are present, do not delay an in-person evaluation. Remember that a mammogram should not be rescheduled because of a vaccination. [50]

Forecast

In most cases (reactive and vaccine-associated nodules, mild infectious episodes), the prognosis is excellent: nodules shrink within weeks to several months; with Bartonella, self-limitation is more common, sometimes with the benefit of a short course of azithromycin. Regular communication with the doctor and a clear monitoring plan help reduce anxiety. [51]

Unfavorable scenarios include missed bacterial abscesses (requiring drainage) or delayed oncological diagnosis. Adherence to the "observe judiciously - examine at risk" algorithm minimizes risks and unnecessary interventions. In oncology, modern selective approaches to the axilla improve quality of life without compromising efficacy. [52]

FAQ

• 1) How long should I wait before the examination?

If the nodule is soft/movable, there is a clear cause (a wound on the arm, a recent vaccination), and there are no "flags," observation for 2-4 weeks is acceptable. If it grows, lasts for more than 4-6 weeks, or has any alarming signs, an ultrasound and, if necessary, a biopsy are recommended. [53]

• 2) Should I postpone my mammogram because of the vaccination?

No. Screening is not postponed; it is important to report the date and side of the vaccination, and if the enlargement of the nodes persists for >3 months, perform a follow-up ultrasound. [54]

• 3) Lymph node "due to nerves" - does this happen?

Stress alone does not cause a lump to enlarge. More often, the discovery is associated with a simple skin reaction or vaccination. However, if the lump is hard and growing, has "B" symptoms, or is a palpable mass in the breast, an examination is necessary. [55]

• 4) Do antibiotics help “just in case”?

No. Antibiotics do not accelerate regression in reactive/vaccine-associated nodes. They are only indicated in cases of obvious bacterial lymphadenitis or purulent complications. [56]

• 5) What is elastography and CEUS - is it better than biopsy?

These are ultrasound methods that assess the stiffness and vascular pattern of a nodule; when combined, they improve the accuracy of distinguishing benign from malignant nodules. However, if a tumor is suspected, the "gold standard" is morphological verification (biopsy). [57]