Lymphomas are a heterogeneous group of lymphoid malignancies that arise primarily in lymph nodes or extranodal lymphoid tissues. They are broadly classified into Hodgkin lymphoma (HL, ~10%) and non-Hodgkin lymphoma (NHL, ~90%), with the latter comprising over 60 distinct entities by WHO classification. The laboratory plays a key role through morphologic assessment, immunophenotyping (flow cytometry and immunohistochemistry), and molecular studies.
Classification Overview
The WHO classification of lymphoid neoplasms categorizes lymphomas by cell of origin: mature B-cell neoplasms (most common), mature T-cell and NK-cell neoplasms, and Hodgkin lymphoma. Each entity is defined by a combination of morphology, immunophenotype, genetic features, and clinical presentation. The peripheral blood may be involved in some lymphomas (leukemic phase), but definitive diagnosis typically requires a lymph node or extranodal tissue biopsy.
Hodgkin Lymphoma
Hodgkin lymphoma is characterized by the presence of Reed-Sternberg (RS) cells and their variants in a characteristic inflammatory microenvironment. RS cells are large with bilobed or multilobed nuclei, prominent nucleoli (owl-eye appearance), and abundant cytoplasm, staining positive for CD15 and CD30, with weak or absent CD45 (LCA). The background is composed of T cells, eosinophils, plasma cells, histiocytes, and fibroblasts. Nodular lymphocyte-predominant Hodgkin lymphoma expresses B-cell markers (CD20, CD79a) and lacks CD15/CD30, with popcorn cells (LP cells) instead of classic RS cells.
Nodular sclerosis (70% of HL) is the most common subtype, presenting with mediastinal mass in young adults, with lacunar cells and collagen band sclerosis. Mixed cellularity (20–25%) occurs in older patients and in HIV, with classic RS cells in a mixed inflammatory background. Lymphocyte-rich (< 5%) has rare RS cells with abundant lymphocytes. Lymphocyte-depleted (< 1%) is aggressive, with numerous RS cells and few lymphocytes, associated with HIV and poor prognosis. EBV is detected in about 40% of HL cases, more commonly in mixed cellularity and HIV-associated disease.
Diffuse Large B-Cell Lymphoma (DLBCL)
DLBCL is the most common NHL (30–40% of adult cases), presenting as a rapidly enlarging nodal or extranodal mass. Morphology: large cells (≥ 2× normal lymphocyte) with vesicular nuclei, prominent nucleoli, and basophilic cytoplasm, with diffuse effacement of architecture. Immunophenotype: CD20+, CD22+, CD79a+, PAX5+, and BCL6+, with variable MUM1/IRF4, BCL2, and MYC expression. Cell-of-origin classification (by gene expression profiling or IHC algorithms): germinal center B-cell (GCB, ~50%, better prognosis) and activated B-cell (ABC, ~30%, worse prognosis). MYC, BCL2, and/or BCL6 rearrangements (double-hit or triple-hit lymphoma) define high-grade B-cell lymphoma with aggressive behavior requiring intensive chemotherapy.
Follicular Lymphoma
Follicular lymphoma (FL) is the second most common NHL (20–30%), typically presenting with painless, slowly progressive lymphadenopathy. Morphology: neoplastic follicles composed of centrocytes (small cleaved cells) and centroblasts (large non-cleaved cells), graded 1–3 by centroblast count. Immunophenotype: CD20+, CD19+, CD10+, BCL2+, BCL6+, with surface immunoglobulin (IgM). The hallmark t(14;18)(q32;q21) IGH-BCL2 rearrangement is present in 85–90% of cases, leading to constitutive BCL2 expression and apoptosis resistance. FL is indolent but incurable, with median survival 8–12 years. Transformation to DLBCL occurs in 2–3% per year.
Mantle Cell Lymphoma
Mantle cell lymphoma (MCL) is a mature B-cell neoplasm with aggressive behavior but incurable course. Lymphocytosis and splenomegaly are common. Morphology: small to medium lymphocytes with irregular nuclei, sometimes with blastoid or pleomorphic variants. Immunophenotype: CD20+, CD5+, cyclin D1+, CD23− (distinguishing it from CLL). t(11;14)(q13;q32) CCND1-IGH rearrangement causes cyclin D1 overexpression. SOX11 is a sensitive nuclear marker for cyclin D1-negative MCL. TP53 mutation and blastoid morphology are high-risk features.
Burkitt Lymphoma
Burkitt lymphoma (BL) is a highly aggressive B-cell neoplasm, one of the fastest-growing human tumors, with three clinical variants: endemic (equatorial Africa, EBV-associated, jaw or facial bone tumors), sporadic (rest of world, abdominal tumors), and immunodeficiency-associated (HIV). Morphology: medium-sized cells with basophilic cytoplasm, cytoplasmic vacuoles (lipid droplets), and a starry-sky pattern (tingible body macrophages). The Ki-67 proliferation fraction is nearly 100%. Immunophenotype: CD20+, CD10+, BCL6+, BCL2−, and MYC+. t(8;14) MYC-IGH is the genetic hallmark.
T-Cell Lymphomas
Peripheral T-cell lymphoma, not otherwise specified (PTCL-NOS) is the most common T-cell lymphoma (25–30% of T-cell NHL), with heterogeneous morphology and aggressive behavior. Anaplastic large cell lymphoma (ALCL) expresses CD30 and ALK in most pediatric cases, with t(2;5) NPM1-ALK. Angioimmunoblastic T-cell lymphoma (AITL) presents with generalized lymphadenopathy, fever, rash, and polyclonal hypergammaglobulinemia, with EBV-positive B blasts in the background. Adult T-cell leukemia/lymphoma (ATLL) is caused by HTLV-1, with characteristic flower cells on blood smear, hypercalcemia, and aggressive course.
Laboratory Diagnosis
Initial evaluation for suspected lymphoma includes complete blood count with differential, serum LDH (prognostic marker), and protein electrophoresis (screening for plasma cell disorders). Peripheral blood smear may show circulating lymphoma cells (leukemic phase). Excisional lymph node biopsy is the gold standard for diagnosis, as core needle biopsy provides limited tissue for full characterization. Immunohistochemistry on paraffin-embedded tissue provides the immunophenotypic profile. Flow cytometry on cell suspensions or fine needle aspirates is complementary. Cytogenetics and FISH detect diagnostic rearrangements. Molecular studies (PCR for IGH/TCR rearrangement) confirm clonality. CSF examination is performed when central nervous system involvement is suspected.
