Lymphadenopathy — Reactive vs Neoplastic

Reactive Lymphadenopathy Mechanisms

• Antigen-driven lymphocyte activation: Lymph nodes respond to pathogenic or foreign antigens by activating resident dendritic cells and T cells within paracortical areas (T-cell zones), followed by B-cell activation in germinal centers. This leads to proliferation of small lymphocytes, plasma cells, and macrophages, creating histological hyperplasia while maintaining normal nodal architecture with preserved cortical-medullary differentiation.

• Preserved germinal center formation: Reactive nodes demonstrate prominent secondary germinal centers with active tingible-body macrophages, polarization into light and dark zones, and a mantle zone of small B lymphocytes. The germinal center reaction represents T-cell–dependent B-cell activation, with somatic hypermutation and selection of high-affinity antibody-producing clones—hallmark features absent in neoplastic processes.

• Architectural preservation with follicular hyperplasia: The nodal capsule remains intact, sinuses are patent and contain histiocytes, and the basic nodal geography is maintained. Paracortical hyperplasia (reactive T-cell zones) and interfollicular hyperplasia may accompany follicular hyperplasia depending on the antigenic stimulus (viral infections favor paracortical reaction; bacterial infections favor follicular hyperplasia).

Neoplastic Lymphadenopathy Mechanisms

• Clonal proliferation and loss of polyclonality: Malignant lymph node enlargement arises from monoclonal expansion of neoplastic lymphocytes with identical immunoglobulin (B-cell lymphomas) or T-cell receptor (T-cell lymphomas) gene rearrangements. Flow cytometry reveals abnormal immunophenotypes (e.g., single light chain restriction in B-cell lymphoma, aberrant T-cell antigen loss in T-cell lymphoma) rather than the polyclonal mixture seen in reactive nodes.

• Architectural effacement and germinal center obliteration: Neoplastic cells progressively destroy normal nodal anatomy, replacing normal lymphocyte populations and obliterating recognizable germinal centers. Loss of capsular integrity and extra-nodal extension may occur. Hodgkin lymphoma exhibits the distinctive Reed-Sternberg cell and Hodgkin cell in a background of small lymphocytes, with fibrosis and necrosis potentially creating nodular sclerosis architecture.

• Impaired apoptosis and cell cycle dysregulation: Neoplastic cells evade programmed cell death through overexpression of anti-apoptotic proteins (BCL-2 in follicular lymphoma; EBV-driven LMP-1 signaling in Hodgkin lymphoma) and acquire activating mutations in cell cycle regulators (TP53, RB, cyclin-CDK pathways). Loss of normal checkpoint controls allows persistence of cells that would be eliminated in reactive processes.

Reactive (Benign) Causes

• Infectious etiologies: Viral infections (EBV, CMV, HIV, hepatitis A/C, dengue) typically cause paracortical hyperplasia; bacterial infections (streptococcal pharyngitis, tuberculosis, cat-scratch disease) and fungal infections (histoplasmosis, toxoplasmosis) cause follicular hyperplasia; atypical presentations may occur with mycobacterial (TB lymphadenitis with caseating granulomas) and parasitic infections.

• Inflammatory and autoimmune diseases: Rheumatoid arthritis, systemic lupus erythematosus (SLE), and drug reactions (phenytoin causing pseudolymphoma) trigger germinal center hyperplasia; granulomatous inflammation occurs in sarcoidosis and berylliosis.

• Medications and toxins: Phenytoin causes atypical lymphoid hyperplasia mimicking lymphoma (DRESS syndrome—Drug Reaction with Eosinophilia and Systemic Symptoms); allopurinol, antibiotics, and NSAIDs are common culprits; silicosis causes anthracosilicosis with regional lymphadenopathy.

Neoplastic Causes

• Hodgkin lymphoma: Presents with mediastinal, supraclavicular, and inguinal lymphadenopathy, often in young adults (bimodal: peaks at 20s and 55+). Classic histology shows Reed-Sternberg cells (multinucleated giant cells with prominent nucleoli and "owl's eye" appearance) in a rich inflammatory background.

• Non-Hodgkin lymphomas (B-cell predominant): Follicular lymphoma (t(14;18) translocation with BCL-2 overexpression), diffuse large B-cell lymphoma (DLBCL), small lymphocytic lymphoma/chronic lymphocytic leukemia (SLL/CLL), and Burkitt lymphoma (t(8;14) with MYC rearrangement) present with nodal enlargement; often multifocal and associated with systemic symptoms or cytopenias.

• T-cell lymphomas and Hodgkin variants: Peripheral T-cell lymphomas (angioimmunoblastic T-cell lymphoma, nodal T-cell lymphomas), cutaneous T-cell lymphomas (mycosis fungoides with nodal involvement), and lymphoma-like disorders.

• Secondary malignancies: Metastatic carcinoma (squamous cell carcinoma from lung/head-neck, adenocarcinoma), sarcomas, and hematopoietic malignancies (acute and chronic leukemias, multiple myeloma) may present with secondary lymphadenopathy.

Cardinal Features of Reactive Lymphadenopathy

• Constitutional symptoms typically absent or mild: Fever and malaise may occur in acute infections but resolve with treatment; lack of B symptoms (fever, night sweats, weight loss) is a key distinguishing feature. When present, symptoms correlate with the underlying infection or inflammation rather than the lymph node burden itself.

• Tender, mobile nodes: Reactive nodes are typically non-matted (freely movable), tender to palpation (due to inflammation and capsular stretch), and show rapid onset and resolution (days to weeks for viral infections, months for chronic processes). Size rarely exceeds 2-3 cm in any dimension.

• Regional distribution pattern: Nodes enlarge in areas draining the source of infection or inflammation—cervical lymphadenopathy follows upper respiratory infections, inguinal adenopathy follows lower extremity or genital infections, and mediastinal lymphadenopathy may follow pulmonary infections.

Cardinal Features of Neoplastic Lymphadenopathy

• B symptoms (constitutional symptoms): Fever, night sweats (drenching, requiring change of clothes), and unintentional weight loss (>10% body weight over 6 months) represent neoplastic systemic effects through tumor necrosis factor-alpha (TNF-α) and interleukin-6 (IL-6) secretion by malignant cells and reactive inflammatory cells. These symptoms correlate with tumor burden and are staging criteria (B symptom vs. A symptom disease).

• Hard, fixed, matted nodes: Neoplastic nodes are characteristically non-tender, firm to hard, and may become matted together or fixed to skin or deep structures as disease progresses and capsular integrity is lost. Progressive enlargement over weeks to months despite appropriate treatment of any identified infection is a red flag.

• Supraclavicular or mediastinal involvement: Supraclavicular lymphadenopathy is particularly suspicious for malignancy (especially Hodgkin or testicular primary malignancy), as these nodes have lower reactive disease rates. Mediastinal mass is highly associated with Hodgkin lymphoma, primary mediastinal B-cell lymphoma, and T-cell lymphoblastic lymphoma.

Laboratory and Imaging Correlates

• Reactive processes: Lab studies show polyclonal immunoglobulin on serum protein electrophoresis, normal or mildly elevated LDH, normal CBC or mild lymphocytosis, and negative malignancy markers. Imaging (ultrasound, CT) demonstrates nodes with preserved hilar architecture, uniform echogenicity, and rapid resolution on follow-up.

• Neoplastic processes: Labs reveal monoclonal protein (in some lymphomas), markedly elevated LDH (particularly in Burkitt and DLBCL), cytopenias (anemia, thrombocytopenia from marrow involvement), and elevated uric acid (from tumor lysis). Imaging shows loss of normal architecture, homogeneous enhancement, and progressive enlargement or multifocal involvement.

Histological Distinction (Gold Standard)

• Reactive hyperplasia features: Preserved nodal capsule and hilar architecture, prominent germinal centers with light and dark zones, tingible-body macrophages (appearing as "starry sky" pattern when numerous), intact mantle zone surrounding germinal centers, paracortical hyperplasia (expanded T-cell areas) with preserved sinuses containing histiocytes. Flow cytometry shows polyclonal B-cell and T-cell populations with normal antigen expression; T cells predominate in paracortical hyperplasia. T-cell and B-cell receptor gene rearrangement studies are polyclonal (multiple clones detected) or show small clones (often <5% clonality, considered background or reactive).

• Neoplastic lymphoma features: Architectural effacement with loss of normal nodal anatomy, effaced or absent germinal centers, monomorphic infiltrate of abnormal lymphocytes (vs. the polymorphic mixture of reactive nodes), and abnormal immunophenotype on flow cytometry (e.g., CD5+ and CD19+ B cells lacking CD23 suggests mantle cell lymphoma; CD10+ and CD19+ B cells suggests follicular lymphoma; aberrant T-cell antigen loss suggests T-cell lymphoma). Monoclonal immunoglobulin or T-cell receptor gene rearrangement (single clonal peak) confirms clonality. Cytomorphology reveals increased mitotic figures, abnormal mitoses, or specific diagnostic cells (Reed-Sternberg cells in Hodgkin lymphoma, Auer rods in acute leukemia, basket-weave nuclei in mycosis fungoides).

Gross Pathology

• Reactive nodes: Yellow-tan cut surface with preserved white matter (cortex), pink-tan medullary cords, and patent sinuses. Nodes are soft to rubbery, with intact capsule. Size typically <2-3 cm. Hilar fat and vasculature are preserved.

• Neoplastic nodes: Fish-flesh appearance with loss of normal architecture, often white-gray or tan color depending on cellularity, bulging cut surface with effaced medullary cords. Capsular thickening and adhesion to surrounding tissues may be present. Necrosis and hemorrhage may be apparent as yellow or dark red areas. Size can be large (>5 cm).

Laboratory Findings

• Reactive: CBC with mild lymphocytosis (absolute lymphocyte count typically <4,000), normal or <2× upper normal limit LDH, normal immunoglobulin levels or polyclonal elevation, negative monospot (in EBV infection, initially negative IgM anti-EBV in early infection), normal uric acid, normal calcium.

• Neoplastic: Markedly elevated LDH (correlates with tumor burden and poor prognosis, especially in Burkitt—LDH >1000 is poor prognosis), possible monoclonal protein on serum/urine electrophoresis, cytopenias (hemoglobin <10 g/dL, platelets <100,000, WBC abnormalities), elevated uric acid and phosphate (from tumor lysis, especially Burkitt and acute lymphoblastic leukemia), hypercalcemia in some lymphomas (especially Hodgkin), elevated B2-microglobulin (worse prognosis marker).

Diagnostic Imaging

• Ultrasound: Reactive nodes are ovoid with preserved hilar echogenicity ("bright hilum"), uniform hypoechogenicity, and short axis <5 mm; neoplastic nodes lose hilar fat, become round or irregular, and show homogeneous echogenicity.

• CT and PET-CT: FDG-PET/CT is highly specific for malignancy; reactive nodes show minimal uptake (SUVmax <2.5), while lymphoma nodes show high standardized uptake values (SUV), particularly in Hodgkin and aggressive non-Hodgkin lymphomas. SUVmax correlates with prognosis. CT shows nodal size, location, and involvement of extranodal sites; staging is per Cotswolds modification of Ann Arbor staging (Stage I—single nodal region; II—two or more regions same side diaphragm; III—nodes both sides diaphragm; IV—extranodal involvement).

Diagnostic Approach

1. 1.Clinical assessment: History of infection, timeline of enlargement, constitutional symptoms, exam findings (tender vs. hard, mobile vs. fixed, regional vs. generalized).
2. 2.Lab workup: CBC, CMP, LDH, reticulocyte count, peripheral blood smear, EBV serology (if clinically indicated), rapid streptococcal test for pharyngitis.
3. 3.Imaging: Ultrasound for initial characterization; CT chest/abdomen/pelvis for staging if malignancy suspected; PET-CT for lymphoma staging.
4. 4.Biopsy indication: Lymph node biopsy is indicated if nodes persist >4-6 weeks despite treatment, rapidly enlarge, are supraclavicular, show imaging features concerning for malignancy, have constitutional symptoms, or affect prognosis/management. Fine-needle aspiration (FNA) cytology has lower diagnostic accuracy and should not be used for primary lymphoma diagnosis; core needle biopsy or excisional biopsy are preferred.
5. 5.Flow cytometry and gene rearrangement studies: Performed on fresh tissue (should be done on biopsy specimens, not FNA).

Reactive Lymphadenopathy

• Treat underlying cause: Antibiotics for bacterial infection (e.g., amoxicillin for streptococcal pharyngitis), antiviral therapy for EBV (supportive care primarily; acyclovir only in severe immunosuppressed patients), antituberculous therapy for TB lymphadenitis (RIPE: Rifampin, Isoniazid, Pyrazinamide, Ethambutol), discontinuation of offending medication (e.g., phenytoin withdrawal in DRESS syndrome). Constitutional symptoms and lymphadenopathy typically resolve with specific treatment of the underlying etiology.

• Supportive care: NSAIDs for discomfort and fever, rest, hydration, and observation for resolution. Most reactive lymphadenopathy resolves within 4-6 weeks of appropriate treatment; this is the key endpoint distinguishing reactive from neoplastic causes.

• No antineoplastic therapy indicated: Chemotherapy or radiation is inappropriate and potentially harmful, as reactive nodes will regress spontaneously. Unnecessary biopsy and treatment should be avoided through clinical judgment.

Neoplastic Lymphadenopathy (Overview; specific regimens based on subtype)

• First-line chemotherapy: ABVD (Doxorubicin, Bleomycin, Vinblastine, Dacarbazine) is standard for classical Hodgkin lymphoma; modern protocols often add involved-field radiation therapy (IFRT) or escalate to more intensive chemotherapy (Stanford V or dose-

Complications of the underlying disease

• Superior vena cava syndrome (emergency): bulky anterior mediastinal nodal mass (classically Hodgkin lymphoma, primary mediastinal B-cell lymphoma, T-lymphoblastic lymphoma) compresses the thin-walled SVC. Signaled by facial/upper-extremity edema, plethora, distended neck and chest wall veins, and orthopnea. Coexisting tracheobronchial compression makes sedation or general anesthesia for biopsy hazardous — obtain cross-sectional imaging first and favor a biopsy approach under local anesthesia.
• Tumor lysis syndrome (emergency): massive cell turnover in high-grade lymphomas (Burkitt, lymphoblastic, bulky DLBCL) releases intracellular contents. Signaled by hyperkalemia, hyperphosphatemia, hyperuricemia, hypocalcemia and acute kidney injury, often before or within hours of the first chemotherapy dose. Managed with aggressive IV hydration plus a xanthine oxidase inhibitor (allopurinol) or recombinant urate oxidase (rasburicase) for high-risk disease; rasburicase is contraindicated in G6PD deficiency.
• Malignant spinal cord compression (emergency): epidural or vertebral nodal extension. Signaled by progressive back pain worse when recumbent, then weakness and sphincter dysfunction — urgent MRI plus corticosteroids (dexamethasone).
• Marrow infiltration and mass effect: cytopenias with infection or bleeding; retroperitoneal nodes causing hydronephrosis; thoracic duct obstruction causing chylothorax or lymphedema.
• Histologic transformation: indolent follicular lymphoma or SLL/CLL (Richter transformation) converts to aggressive large-cell disease. Signaled by one node enlarging disproportionately, new B symptoms, and a sharply rising LDH — rebiopsy that node.
• Suppurative or fistulizing lymphadenitis: seen with mycobacterial (scrofula), cat-scratch, and fungal disease; signaled by fluctuance and draining sinus tracts.

Treatment-related complications

• Bleomycin pulmonary toxicity: oxidant lung injury; signaled by dry cough, dyspnea and a falling DLCO — NCCN survivorship guidance supports pulmonary monitoring and prompt drug discontinuation.
• Anthracycline cardiomyopathy: cumulative, dose-dependent; signaled by a fall in LVEF on surveillance echocardiography.
• Hepatitis B reactivation with rituximab: B-cell depletion permits viral escape; AASLD and ASCO advise HBsAg/anti-HBc screening and antiviral prophylaxis before anti-CD20 therapy. Fulminant hepatitis is an emergency.
• Second malignancy and endocrine injury: alkylator/topoisomerase-related MDS-AML, and post-chest-radiation breast cancer and hypothyroidism — NCCN survivorship protocols drive long-term screening.

• Excisional biopsy is the single best next step for a persistently enlarging, firm, non-tender node — FNA cannot show architecture, and architectural effacement is the diagnosis. Do not give empiric corticosteroids before tissue is obtained: steroids lyse lymphoid cells and can render the specimen non-diagnostic. This is the most commonly tested distractor in the stem.
• Supraclavicular adenopathy is malignant until proven otherwise. Left supraclavicular = Virchow node, drainage from the abdomen (gastric, pancreatic, testicular primaries). The classic distractor is the Sister Mary Joseph nodule, which is periumbilical, not supraclavicular.
• **Alcohol-induced nodal pain and the Pel-Ebstein cyclic fever point to Hodgkin lymphoma. Reed-Sternberg cells are CD15+, CD30+, CD20− and CD45−**; the distractor answer is CD45 positivity, which characterizes non-Hodgkin lymphomas.
• Translocations examiners recycle: t(14;18) BCL-2 → follicular lymphoma (impaired apoptosis); t(8;14) MYC → Burkitt (starry sky, tingible-body macrophages amid a monomorphic sheet — distinguish from the tingible-body macrophages of a normal reactive germinal center); t(11;14) cyclin D1 → mantle cell (CD5+, CD23−, versus CD5+/CD23+ CLL/SLL).
• The atypical lymphocytes of EBV mononucleosis are reactive CD8+ T cells, not infected B cells — a paracortical (T-zone) hyperplasia. Avoid aminopenicillins (morbilliform rash) and advise avoiding contact sports because of splenic rupture risk.
• Drug-induced pseudolymphoma: phenytoin (and DRESS) produces atypical lymphoid hyperplasia that mimics lymphoma histologically; the intervention is stopping the drug, and polyclonality on flow cytometry settles it.
• Granulomatous patterns: stellate necrotizing granulomas with microabscesses = cat-scratch disease (Bartonella henselae, azithromycin); caseating granulomas = tuberculous lymphadenitis; non-caseating granulomas with bilateral hilar adenopathy = sarcoidosis.
• Clonality is the dividing line: light-chain restriction or a single immunoglobulin/T-cell receptor rearrangement peak means neoplastic; a polyclonal mixture with preserved capsule, patent sinuses and mantle zones means reactive.