Lymphoma diagnosis depends on what pathologists see when they examine tissue under a microscope, combined with a growing toolkit of stains, molecular probes, and genetic tests applied to that same tissue. Unlike many solid tumors where a pathologist can often render a diagnosis from basic staining alone, lymphomas demand layers of investigation because the cells involved are immune cells that can look deceptively similar to one another and even to normal tissue reactions. The microscopic view is where diagnosis begins, but the journey from a tissue slide to a confident lymphoma subtype is more involved than most patients realize.
Getting the Right Tissue
Everything starts with the biopsy. The gold standard for lymphoma diagnosis has long been a surgical excisional biopsy, where a surgeon removes an entire lymph node. This gives the pathologist a large, intact piece of tissue with preserved architecture, which matters enormously because the pattern of how cells are arranged often matters as much as the cells themselves. A follicular growth pattern versus a diffuse one, for instance, can mean the difference between an indolent and an aggressive disease.
Core needle biopsy has become a practical alternative, especially when surgery poses risks or a deep lymph node is hard to reach. A study comparing the two approaches found that surgical excisional biopsy achieved a full diagnostic rate of about 95%, while core needle biopsy reached roughly 84%. Yet when the question was narrowed to whether the pathologist had enough information to start treatment without a second biopsy, the gap nearly closed: around 93% for core needle versus 95% for excision.1PubMed Central. Should Core Needle Lymph Node Biopsy be a Relevant Alternative to Surgical Excisional Biopsy in Diagnostic Work Up of Lymphomas? A separate analysis put the diagnostic accuracy of core needle biopsy at about 91% compared with 97% for excision, confirming a real but manageable gap.2PubMed. Core Needle Biopsy in Lymphoma Diagnosis: The Diagnostic Performance and the Role of the Multidisciplinary Approach in the Optimization of Results Where core needle biopsy falls short is in cases where architecture is critical or where small sample size limits the number of ancillary tests that can be run. Crush artifact from the needle procedure itself can also distort cells in fibrotic tissues, making interpretation harder.
The First Look Under the Microscope
Once tissue is processed, the pathologist’s initial tool is a standard hematoxylin and eosin (H&E) stain. Hematoxylin stains cell nuclei a dark blue-purple, while eosin colors the surrounding cytoplasm and connective tissue pink. This basic two-color contrast reveals how cells are organized, how big they are, what their nuclei look like, and how many are dividing. An experienced hematopathologist can often narrow the possibilities dramatically from the H&E slide alone, even if a definitive subtype requires more testing.
At low magnification, the pathologist first evaluates the overall architecture. Is the normal lymph node structure preserved, partially effaced, or completely replaced? Are there recognizable follicles, or has the tissue been taken over by sheets of abnormal cells? These broad patterns immediately suggest categories. At higher magnification, individual cell features come into focus: nuclear size, the shape and prominence of nucleoli, the amount of cytoplasm, and the mitotic rate. All of these details point toward specific diagnoses, which is why the tissue quality from the biopsy matters so much.
Hodgkin Lymphoma Under the Microscope
Classic Hodgkin lymphoma is one of the most distinctive things a pathologist can see on a slide, and also one of the strangest. The malignant cells, called Hodgkin and Reed-Sternberg (HRS) cells, are large and atypical, often with two mirror-image nuclei and big, prominent nucleoli. This gives them a look that pathologists have described as “owl’s eye” for well over a century. Variants of these cells include lacunar cells (which appear to sit in clear spaces due to tissue processing), mummified cells (which look shrunken and darkly stained), and mononuclear Hodgkin cells with a single large nucleus.3PubMed Central. Hodgkin Reed-Sternberg Cells of Classic Hodgkin Lymphoma: Morphology, Phenotype, Genotype, and Cell of Origin
What makes Hodgkin lymphoma so peculiar is that the actual tumor cells are a tiny minority of what you see on the slide. The overwhelming majority of cells in a Hodgkin lymphoma biopsy are non-malignant inflammatory and immune cells: T cells, B cells, eosinophils, plasma cells, and macrophages. The HRS cells recruit and reprogram this surrounding cellular neighborhood, called the tumor microenvironment, using a barrage of signaling molecules. These normal bystander cells are essentially hijacked into supporting the survival and growth of the rare malignant cells.4PubMed Central. Hodgkin Lymphoma: A Special Microenvironment The composition and arrangement of this microenvironment are what define the four subtypes of classic Hodgkin lymphoma: nodular sclerosis (the most common, characterized by bands of fibrous tissue and lacunar cells), mixed cellularity, lymphocyte-rich, and lymphocyte-depleted.
Because HRS cells are so sparse, confirming the diagnosis with immunohistochemistry is standard practice. Classic Hodgkin lymphoma cells typically stain positive for CD15 and CD30, show weak expression of the B-cell transcription factor PAX5, and are negative for most other B-cell and T-cell markers as well as CD45.5ScienceDirect / Human Pathology. Classic Hodgkin lymphoma: An illustrative review of select diagnostic limitations and immunomorphological challenges This immunophenotype is shared across all four subtypes and is a powerful way to distinguish Hodgkin from non-Hodgkin lymphoma, which matters enormously because the treatments are different.
Non-Hodgkin B-Cell Lymphomas
Non-Hodgkin lymphomas are far more heterogeneous. The majority arise from B cells, and the microscopic features of each subtype reflect where in the B-cell maturation process the cancer originated. A few of the most common subtypes illustrate how much variety exists under a single diagnostic umbrella.
Diffuse Large B-Cell Lymphoma
Diffuse large B-cell lymphoma (DLBCL) is the most common aggressive lymphoma worldwide. Under the microscope, it shows sheets of large lymphoid cells growing in a diffuse pattern that obliterates normal architecture. The cells have prominent nuclei, visible nucleoli, and a moderate amount of cytoplasm. What the H&E slide alone cannot tell you is the cell of origin, which has implications for prognosis and treatment. Gene expression profiling can classify DLBCL into germinal center B-cell (GCB) and activated B-cell (ABC) subtypes, but most pathology labs use immunohistochemical algorithms as a practical stand-in. The Hans algorithm, one of the most widely used, correctly identifies the GCB subtype with a sensitivity of about 83% and the non-GCB subtype with a sensitivity of about 70%.6Blood. Cell of Origin Determination in Diffuse Large B-Cell Lymphoma: Performance of Immunohistochemical (IHC) Algorithms and Ability to Predict Outcome No single protein marker can reliably mirror gene expression classification on its own, which is why panels of multiple stains are used together.7Clinical Cancer Research. Poor Concordance among Nine Immunohistochemistry Classifiers of Cell-of-Origin for Diffuse Large B-Cell Lymphoma: Implications for Therapeutic Strategies
One important marker in DLBCL is Ki-67, which stains cells that are actively dividing. DLBCL typically shows Ki-67 positivity in the range of 40% to 90%, which is high but can overlap with even more aggressive entities. Burkitt lymphoma, for instance, characteristically pushes Ki-67 close to 100%, and this distinction matters for treatment planning.8PubMed Central. Ki-67 as a Marker to Differentiate Burkitt Lymphoma and Diffuse Large B-cell Lymphoma: A Literature Review
Follicular Lymphoma
Follicular lymphoma is the most common indolent B-cell lymphoma and usually presents a nodular (follicular) growth pattern that can, at first glance, mimic the normal follicles of a reactive lymph node. The neoplastic follicles are composed of two cell types, centrocytes (small, irregular, cleaved cells) and centroblasts (larger cells with open nuclei and multiple nucleoli), in variable proportions.9PubMed Central. The broad landscape of follicular lymphoma: Part I The ratio of centroblasts to centrocytes is the basis for grading follicular lymphoma: the more centroblasts per high-power microscopic field, the higher the grade, and higher grades are associated with more aggressive behavior.10Journal of Pathology and Translational Medicine. Follicular lymphoma: updates for pathologists
Roughly 85% of follicular lymphomas carry a characteristic chromosomal translocation, t(14;18), which fuses the BCL2 gene to the immunoglobulin heavy chain locus. The result is overexpression of the BCL2 protein, which blocks programmed cell death.11PubMed Central. Follicular Lymphoma in the 5th Edition of the WHO-Classification of Haematolymphoid Neoplasms-Updated Classification and New Biological Data BCL2 immunostaining is one of the most useful tools for distinguishing neoplastic follicles from reactive follicular hyperplasia, because normal germinal center B cells are typically BCL2-negative. When neoplastic follicles light up strongly for BCL2, the pathologist has a powerful argument that this is lymphoma rather than a benign reaction.
Mantle Cell Lymphoma
Mantle cell lymphoma (MCL) gets its name from the mantle zone of the lymphoid follicle, where its cells originate. Under the microscope, MCL classically shows a monotonous proliferation of small to medium-sized lymphoid cells with irregular nuclear contours. In a study of 168 cases, the most common pattern was nodular (about 54% of cases), followed by diffuse (about 46%), with recognizable mantle zone patterns in a smaller subset.12PubMed. Morphological spectrum of cyclin D1-positive mantle cell lymphoma: study of 168 cases The hallmark molecular feature is overexpression of cyclin D1, a cell-cycle protein, driven by the translocation t(11;14). Cyclin D1 immunostaining is the main diagnostic anchor.
MCL can fool pathologists because its morphologic range is wider than the textbook image suggests. At one end are small-cell variants that resemble chronic lymphocytic leukemia; at the other are blastoid variants with high mitotic rates and aggressive behavior. The blastoid forms show markedly elevated proliferation compared to the common type and frequently carry additional genetic abnormalities including p53 overexpression and chromosome numbers in the tetraploid range.13Blood. Blastoid Variants of Mantle Cell Lymphoma: Frequent bcl-1 Rearrangements at the Major Translocation Cluster Region and Tetraploid Chromosome Clones
T-Cell Lymphomas and Their Diagnostic Challenges
T-cell lymphomas are less common than their B-cell counterparts but generally harder to diagnose histologically. Part of the difficulty is that they lack a convenient equivalent of light-chain restriction, the clonality marker that pathologists rely on for B-cell tumors. Instead, T-cell lymphomas are suspected when the pathologist notices abnormal-looking T cells and, critically, when those cells have lost one or more of the surface markers that normal T cells carry.
Loss of so-called pan-T-cell antigens is one of the strongest histologic clues. In a study of 50 peripheral T-cell lymphomas, loss of at least one pan-T marker was found in the majority of cases, with individual markers lost in roughly a quarter to nearly half of tumors depending on the specific antigen.14PubMed Central. Morphologic and immunologic characterization of 50 peripheral T-cell lymphomas More recent work has refined this concept by looking at aberrant expression patterns of CD3 subunits, finding that certain CD3 components show abnormal staining in over half of peripheral T-cell lymphomas.15Annals of Diagnostic Pathology. Diagnostic utility of the aberrant immunohistochemical expression of CD3 molecules for peripheral T-cell lymphomas
Anaplastic large cell lymphoma (ALCL) is one T-cell subtype with distinctive microscopic features. Its cells are large, with kidney-shaped or horseshoe-shaped nuclei often described as “hallmark cells.” ALCL is divided into ALK-positive and ALK-negative types based on expression of the ALK protein, which reflects the presence of the ALK gene rearrangement. Among ALK-negative cases, those with DUSP22 rearrangements tend to grow in sheets of hallmark cells with a notable frequency of “doughnut cells” (cells with a ring-shaped nucleus) and relatively few pleomorphic giant cells.16PubMed Central. Morphologic Features of ALK-negative Anaplastic Large Cell Lymphomas With DUSP22 Rearrangements This matters because ALK-negative ALCL with DUSP22 rearrangements behaves more favorably than other ALK-negative cases, so recognizing the morphologic pattern can prompt the right genetic test.
The Immunohistochemistry Panel
Immunohistochemistry (IHC) is where diagnosis shifts from educated pattern recognition to targeted confirmation. Antibodies labeled with colored or fluorescent markers are applied to tissue sections, and each antibody binds to a specific protein on or inside the cells. The staining pattern tells the pathologist what type of cell is present and, in many cases, what subtype of lymphoma they are dealing with.
The starting point is lineage determination. CD20 is the workhorse marker for B cells, and CD3 for T cells. Most mature B-cell lymphomas express CD20, while most mature T-cell and NK-cell lymphomas express CD3, with ALCL being a notable exception that often lacks CD3.17PubMed Central. Basic immunohistochemistry for lymphoma diagnosis Once the lineage is established, additional markers are layered on. A primary diagnostic panel for B-cell non-Hodgkin lymphoma typically includes markers like CD5, CD10, CD23, and kappa/lambda light chains, with an extended panel adding CD30, cyclin D1, BCL2, and BCL6 for further subtyping.18Blood Research. Optimal panel of immunohistochemistry for the diagnosis of B-cell non-Hodgkin lymphoma using bone marrow biopsy: a tertiary care center study A minimal core panel of CD3, CD5, CD20, CD45, and PAX5 can reliably sort cases into B-cell versus T-cell lineage as the first triage step.19PubMed Central. Minimally required essential bio markers for lymphoma
Light-chain restriction is a particularly useful concept for B-cell lymphomas. Normal B cells in a lymph node express a healthy mix of kappa and lambda immunoglobulin light chains. A population overwhelmingly expressing one type over the other implies a clonal, and therefore neoplastic, proliferation. This can be detected by IHC on tissue sections or by flow cytometry on a fresh sample. In situ hybridization for kappa and lambda mRNA offers another route when IHC results are ambiguous.20PubMed Central. Kappa and lambda light chain mRNA in situ hybridization compared to flow cytometry and immunohistochemistry in B cell lymphomas Occasionally a genuine B-cell lymphoma fails to show light-chain restriction on flow cytometry, creating a diagnostic trap that requires correlation with the tissue biopsy.21PubMed. Histopathological analysis of B-cell non-Hodgkin lymphomas without light chain restriction by using flow cytometry
Molecular Tests That Work on the Same Tissue
Molecular techniques have become inseparable from histologic diagnosis, especially for aggressive lymphomas where specific genetic changes define the disease category and dictate treatment intensity. Fluorescence in situ hybridization (FISH) uses fluorescent-labeled DNA probes applied directly to tissue sections to detect chromosomal rearrangements. It is an essential study for identifying rearrangements of MYC, BCL2, and BCL6 in large B-cell lymphomas.22Frontiers in Oncology. Performance of MYC, BCL2, and BCL6 break-apart FISH in small biopsies with large B-cell lymphoma: a retrospective Cytopathology Hematopathology Interinstitutional Consortium study
The importance of these rearrangements is hard to overstate. A large B-cell lymphoma with concurrent MYC and BCL2 or BCL6 rearrangements (often called a “double-hit” lymphoma) is classified as a distinct, aggressive entity with a poor prognosis that usually requires intensified chemotherapy.23PubMed. Lymphomas with pseudo-double-hit BCL6-MYC translocations due to t(3;8)(q27;q24) are associated with a germinal center immunophenotype, extranodal involvement, and frequent BCL2 translocations FISH testing for these rearrangements has become a routine part of the workup for any new DLBCL diagnosis. In some cases, rare chromosomal fusions can create what appear to be double-hit events on FISH but are actually “pseudo-double-hit” rearrangements with different biology, highlighting how molecular results still need pathologist interpretation alongside morphology.
When Reactive Tissue Mimics Lymphoma
One of the most important roles of the pathologist is knowing when not to diagnose lymphoma. Benign reactive conditions can produce tissue patterns that look alarmingly like malignancy. Reactive follicular hyperplasia, for example, produces enlarged follicles that can resemble follicular lymphoma. The key differences include variation in follicle size and shape (neoplastic follicles tend to be more uniform), preservation of mantle zones around reactive follicles, and the BCL2 staining pattern mentioned earlier.
Reactive paracortical hyperplasia, particularly in the setting of Epstein-Barr virus infection (infectious mononucleosis), can produce large, activated immunoblasts that mimic Hodgkin lymphoma or aggressive T-cell lymphoma. Kikuchi necrotizing lymphadenitis, a self-limited inflammatory condition, produces areas of tissue death in lymph nodes that can be confused with non-Hodgkin lymphoma.24Modern Pathology. Benign lymphadenopathies In each of these scenarios, careful attention to the immunohistochemical profile and, when necessary, molecular testing for clonality can resolve the question. A misdiagnosis in either direction carries serious consequences: missing a lymphoma delays life-saving treatment, while overdiagnosing one subjects a patient to unnecessary chemotherapy.
Lymphomas That Arise Outside Lymph Nodes
Not all lymphomas originate in lymph nodes. Extranodal marginal zone lymphoma of mucosa-associated lymphoid tissue, commonly called MALT lymphoma, is the third most common non-Hodgkin lymphoma subtype, accounting for roughly 6–8% of all non-Hodgkin lymphomas in Western countries.25PubMed Central. Mucosa-associated lymphoid tissue (MALT) lymphoma: a practical guide for pathologists These lymphomas arise in organs that do not normally contain lymphoid tissue but have acquired it through chronic immune stimulation. The stomach is the classic site, where chronic Helicobacter pylori infection drives the formation of lymphoid tissue that can eventually transform into lymphoma.
Histologically, MALT lymphoma shows small lymphoid cells infiltrating glandular structures, often with a characteristic “lymphoepithelial lesion” where tumor cells invade and destroy the epithelial glands. The diagnosis and management require integrating morphology, immunohistochemistry, and sometimes molecular studies for specific translocations. Gastric MALT lymphoma confined to the stomach wall can sometimes be cured by eradicating the bacterial infection alone, a remarkable example of how understanding the tissue biology can guide a completely non-chemotherapy approach.
How Classification Has Evolved
The way lymphomas are classified has undergone dramatic revision over the past half century. The earliest scheme, proposed by Rappaport in the 1960s, grouped lymphomas by growth pattern and cell size but was based on incorrect assumptions about the cells of origin. Newer systems in the 1970s, including the Lukes-Collins and Kiel classifications, tried to map each lymphoma to a specific stage of normal lymphoid cell development. The modern era began with the 1994 Revised European-American Lymphoma (REAL) classification, which aimed to define “real” disease entities based on a combination of morphology, immunophenotype, genetics, and clinical behavior. The REAL classification became the template for the World Health Organization classification system first published in 2001.26Seminars in Diagnostic Pathology. Lymphoma classification: the quiet after the storm
Today there are actually two parallel classification systems in use: the 5th edition of the WHO classification and the International Consensus Classification (ICC), both published in 2022. They agree on most points but diverge on certain entity names and diagnostic boundaries, particularly for mature B-cell lymphomas and some T-cell lymphomas.27PubMed Central. A comparison of the International Consensus and 5th World Health Organization classifications of mature B-cell lymphomas28PubMed. A comparison of the International Consensus and 5th WHO classifications of T-cell lymphomas and histiocytic/dendritic cell tumours For pathologists and oncologists, the coexistence of two systems is sometimes a source of confusion, though in practice the clinical impact of their differences is modest for most common diagnoses.
Artificial Intelligence and the Future of the Slide
Digital pathology, where glass slides are scanned into high-resolution whole-slide images, has opened the door for artificial intelligence tools trained to recognize lymphoma patterns. Deep learning models have been trained to distinguish follicular lymphoma from reactive follicular hyperplasia on H&E-stained digital slides, using hundreds of thousands of image patches extracted from whole-slide images across different magnification levels.29npj Digital Medicine. Accurate diagnosis of lymphoma on whole-slide histopathology images using deep learning These models show promise for reducing the subjectivity inherent in human interpretation, particularly for borderline cases and grading decisions where pathologists may disagree.30PubMed Central. Artificial Intelligence in Lymphoma Histopathology: Systematic Review
That said, AI in lymphoma diagnosis remains a research tool more than a clinical one. Lymphoma subtyping is unusually complex because it integrates morphology, immunophenotype, and molecular data in ways that a purely image-based algorithm cannot yet replicate. The realistic near-term role for AI is as a screening assistant, flagging suspicious areas and highlighting features for the pathologist to review, rather than as a replacement for the integrated human judgment that defines the field. A slide that could be follicular lymphoma or florid follicular hyperplasia still needs a pathologist to order BCL2 staining, review the clinical history, and make the call.