From Ambiguity to Concrete Answers: CBC to Flow Cytometry to Cytogenetics in Follicular Lymphoma
Published 2026-10-15
The specimen hits the lab with a straightforward, but ambiguous, clinical history. Before flow cytometry or cytogenetics ever sees the tube, the first clue comes from the most ordinary test in the building: the Complete Blood Count.
The CBC raises the alarm. Flow cytometry points the way. Cytogenetics names the enemy. This is the story of how one sample moves through all three, and how each lab hands the next one exactly what it needs.
The short version. A lymphocytosis with small cleaved cells on the smear suggests a circulating mature B-cell neoplasm. Flow cytometry shows a monotypic, CD10-positive, CD5-negative B-cell population, which points at a germinal-center lymphoma. The karyotype finds the reciprocal translocation t(14;18)(q32;q21), and interphase FISH with an IGH::BCL2 dual-color, dual-fusion probe proves the partners. Result: follicular lymphoma, BCL2-driven.
The starting line: reading the CBC
The automated numbers come back first, so start there.
| CBC finding | Term | What it tells you |
|---|---|---|
| White blood cell count increased | Leukocytosis | Something is driving extra white cells into the blood |
| Absolute lymphocyte count increased | Lymphocytosis | The excess is lymphoid |
| RBC, hemoglobin, hematocrit normal | No anemia | The marrow is still making red cells normally |
Look at the absolute lymphocyte count, not just the percentage. A high percentage can simply mean the neutrophils went down. A high absolute number means there really are more lymphocytes in circulation.
Leukocytosis with an absolute lymphocytosis in an adult is a red flag for a lymphoid malignancy: a chronic leukemia such as CLL, or a lymphoma spilling into the blood. It can also be reactive, which is why the next stop is the smear.
When the peripheral smear shows small lymphoid cells with cleaved, notched nuclei (centrocytes), the picture narrows. These are not the large, reactive, abundant-cytoplasm lymphocytes of a viral infection, and they are not the round-nucleus, smudge-cell-heavy picture of classic CLL. The leading suspicion is now a mature B-cell neoplasm, with follicular lymphoma (FL) in a leukemic phase high on the list.
But a CBC and a smear are only the starting line. To turn "abnormal cell count" into an actionable diagnosis, the laboratory runs a relay. The baton passes from flow cytometry (the scout) to G-banded karyotyping (the map) and finally to fluorescence in situ hybridization (the proof).
Step 1: The scout (flow cytometry immunophenotyping)
Before cytogenetics spends hours on banded chromosomes, everyone needs to know what they are looking for. Flow cytometry tags the surface proteins of the abnormal cells and answers two questions:
- Clonality. Is this a neoplasm?
- Phenotype. Which kind?
For a leukemic phase of classic follicular lymphoma, the B-cell panel typically shows:
- CD19+ and CD20+ (bright). These are mature B cells. Bright CD20 is typical of FL and helps separate it from CLL, where CD20 is usually dim.
- Surface light chain restriction. The population expresses only kappa or only lambda. Normal and reactive B cells are polytypic, a mix of both. A monotypic population is the fingerprint of a clone.
- CD10+. The key marker. CD10 (once called CALLA, the common acute lymphoblastic leukemia antigen) is expressed by germinal-center B cells, so a mature, CD10-positive clone points to a germinal-center origin.
- CD5-. Argues against CLL and mantle cell lymphoma, which are typically CD5-positive. (CD5-negative variants of both exist, so this is strong evidence, not an absolute rule.)
| Marker | Follicular lymphoma | CLL/SLL | Mantle cell lymphoma |
|---|---|---|---|
| CD5 | Negative | Positive | Positive |
| CD10 | Positive | Negative | Negative |
| CD20 | Bright | Dim | Bright |
| CD23 | Usually negative | Positive | Usually negative |
| Surface light chain | Moderate to bright, restricted | Dim, restricted | Moderate to bright, restricted |
The handoff. The flow lab reports a "CD10-positive, CD5-negative monotypic B-cell population." To the cytogeneticist, that is the bat-signal. The disease looks germinal-center derived, and the first target to hunt is the BCL2 translocation.
Flow is not the last word on its own. CD10-positive mature B-cell lymphomas also include Burkitt lymphoma and some diffuse large B-cell lymphomas (DLBCL), which is exactly why the genetics matter next.
Step 2: The map (G-banded karyotyping)
With the flow result in hand, the cytogenetics technologist takes over.
Getting B cells to divide
A karyotype needs cells in metaphase, and the standard peripheral blood mitogen, PHA (phytohemagglutinin), stimulates T cells. Culture a B-cell lymphoma with PHA and you will mostly karyotype the patient's normal T cells.
So the culture is set up with B-cell mitogens, such as the CpG oligonucleotide DSP30 combined with interleukin-2 (IL-2), often alongside an unstimulated culture. The goal is to make the malignant clone actually divide and arrest in metaphase.
What you are looking for
Once the slides are G-banded and under the scope, the technologist looks for a specific structural rearrangement: the reciprocal translocation t(14;18)(q32;q21).
- Chromosome 18, band q21 holds BCL2, an anti-apoptotic gatekeeper that keeps cells from dying.
- Chromosome 14, band q32 holds the immunoglobulin heavy chain locus (IGH) and its powerful enhancers.
The distal long arm of 18 (from q21 to the end) is a much bigger piece than the tip of 14 beyond q32. So after the exchange, the der(14) looks longer than a normal 14, and the der(18) looks shorter than a normal 18.
How the accident happens
The translocation is an elegant, if devastating, mistake in the machinery B cells use to build their antibodies.
It happens early, in a precursor B cell in the bone marrow, during V(D)J recombination of the heavy chain gene. The RAG enzymes cut DNA at the IGH joining (JH) segments, and the cell is supposed to rejoin the pieces of its own antibody gene. Instead, a break near BCL2 on chromosome 18 gets joined in. BCL2 ends up sitting next to IGH, under the control of the immunoglobulin enhancers.
There is no new fusion protein. The BCL2 protein is perfectly normal, there is just far too much of it, all the time. That matters because germinal-center B cells are normally supposed to switch BCL2 off and die unless they win the selection contest. The t(14;18) cell never switches it off. When it reaches the germinal center later in its life, it survives where it should have died.
Worth remembering. The lymphoma cells do not necessarily proliferate wildly. They gain "immortality" by evading apoptosis. That is why follicular lymphoma is usually indolent but so hard to eradicate. Also note that small numbers of t(14;18)-positive B cells can be found in many healthy adults, so the translocation alone is not enough to cause lymphoma. Additional genetic hits are needed.
The limits of the karyotype
A skilled cytogeneticist can spot the long der(14) and short der(18). But karyotyping has real limitations here:
- Lymphoma cells are stubborn in culture. Low mitotic index and fuzzy, poorly spread metaphases are common.
- Normal cells may win. If only normal cells divide, the karyotype can look normal even with disease present.
- Subtle or complex rearrangements can be hard to call at the band level, and variant or cryptic rearrangements can be missed.
The karyotype is still valuable: it is the only test in this workup that sees the whole genome at once, including the secondary abnormalities that often travel with t(14;18) and can matter for prognosis and transformation. But it needs a partner.
Step 3: The definitive proof (interphase FISH)
To skip the need for dividing cells and to confirm the exact molecular partners, the lab moves to fluorescence in situ hybridization (FISH) on interphase nuclei. FISH reads every nucleus on the slide, dividing or not, and works on blood, marrow, or fixed tissue sections from a lymph node biopsy.
For t(14;18), the workhorse is the dual-color, dual-fusion (DCDF) IGH::BCL2 probe.
- Green signal: IGH locus, 14q32
- Red (orange) signal: BCL2 locus, 18q21
How to read the dual-fusion probe
| Pattern | What it means |
|---|---|
| 2R2G: two separate red, two separate green | Normal (negative). Two normal copies each of chromosomes 14 and 18. |
| 1R1G2F: one red, one green, two fusion (yellow) | Positive for t(14;18). The two fusions are the der(14) and der(18), where the red and green probes now sit together. The remaining red and green are the normal 18 and 14. |
| Extra fusions or extra red/green signals | Positive, with additional changes. Often a duplicated derivative or gain of a whole chromosome. Common in FL; report what you see. |
Why cut-offs exist. Two signals that happen to land close together in a normal nucleus can look like a fusion. That artefact happens on every probe, which is why no result rests on a single cell. Each laboratory validates a normal cut-off, scores a defined number of nuclei (commonly 200 for an oncology interphase assay), and reports positive only when the abnormal fraction exceeds that cut-off.
Why dual-fusion over break-apart?
A BCL2 break-apart probe brackets the BCL2 gene with two colors. If the colors split, you know BCL2 is rearranged, but not who the partner is.
The DCDF probe answers a sharper question: is IGH specifically the partner? Because flow cytometry already said this is probably follicular lymphoma, the dual-fusion probe is the most direct confirmation. Two fusion signals require both derivatives to carry both probes, which also makes a true positive much harder to produce by chance than a single fusion.
Break-apart probes still earn their place when the partner might not be IGH (variant BCL2 translocations to the IGK or IGL light chain loci exist), or when you are screening a high-grade lymphoma for BCL2 rearrangement alongside MYC and BCL6.
Bench tip. t(14;18) is the hallmark of follicular lymphoma, but it is not exclusive to it. It is also found in a meaningful fraction of DLBCL, and together with a MYC rearrangement it defines a high-grade lymphoma category. The genetics confirm the mechanism; the full diagnosis is always integrated with morphology, phenotype, and the clinical picture.
Writing it up
The karyotype and the FISH report describe the same event in two different dialects of ISCN. For a female patient with the clone in 18 of 20 metaphases and 176 of 200 positive nuclei, it might read:
46,XX,t(14;18)(q32;q21)[18]/46,XX[2]
nuc ish(IGHx3,BCL2x3)(IGH con BCL2x2)[176/200]
The FISH string counts every copy of each probe (the normal 14 and both derivatives each carry IGH signal, and likewise for BCL2), then states that IGH is connected with BCL2 twice: the two fusions. For a refresher on how these strings are built, see the ISCN Nomenclature Cheat Sheet, and for a deeper look at probe designs, FISH Probe Types Explained.
The clinical impact
In the 5th edition of the World Health Organization Classification of Haematolymphoid Tumours (WHO-HAEM5), the framework for follicular lymphoma was refined: classic FL is separated from follicular large B-cell lymphoma, FL with uncommon features, and the BCL2-negative pediatric-type FL. Through all of it, t(14;18) remains the foundational genetic hallmark, present in roughly 85% of classic cases.
Connecting the phenotype (flow cytometry) to the structure (karyotype) and the molecular partners (FISH) gives the oncologist an exact profile, not just a name.
It also explains the biology behind a newer class of drugs. Venetoclax is a BH3-mimetic that binds BCL2 directly and releases the brake on apoptosis. It has transformed treatment of CLL, a disease that also depends heavily on BCL2. In follicular lymphoma, single-agent responses have been more modest, and BCL2 inhibition is still being studied in combinations and clinical trials rather than used as standard therapy. The t(14;18) is the reason it was worth trying in the first place.
Five things to walk away with
| # | Takeaway | Why it matters at the bench |
|---|---|---|
| 1 | Absolute lymphocytosis plus small cleaved cells means "think mature B-cell neoplasm" | The CBC and smear decide which panel and which probes come next |
| 2 | CD10+, CD5-, monotypic B cells point to a germinal-center lymphoma | That phenotype is what tells cytogenetics to hunt for BCL2 |
| 3 | B-cell lymphoma cultures need B-cell mitogens, not PHA | Otherwise you karyotype the normal T cells and miss the clone |
| 4 | t(14;18) is born in the bone marrow, during V(D)J recombination | The cell carries it for years; the germinal center is where it pays off |
| 5 | 2R2G is normal, 1R1G2F is positive, and the cut-off decides | Dual fusion confirms IGH as the partner; break-apart only says BCL2 moved |
The CBC raised the alarm. Flow cytometry pointed the way. But it is the cytogenetics lab that definitively names the enemy.
Studying for the cytogenetics or flow boards? This exact relay, phenotype to karyotype to FISH, is a favorite exam scenario. Practice it with our free cytogenetics practice test, or browse banding on chromosome 14 and chromosome 18 in the visualizer to see exactly where q32 and q21 sit.
Key resources and further reading for the bench
- Arsham MS, Barch MJ, Lawce HJ, eds. The AGT Cytogenetics Laboratory Manual. 4th ed. Wiley-Blackwell; 2017. The standard reference for culture techniques, B-cell mitogen protocols, and FISH troubleshooting.
- Kurz KS, Ott M, Kalmbach S, et al. Follicular lymphoma in the 5th edition of the WHO-classification of haematolymphoid neoplasms: updated classification and new biological data. Cancers (Basel). 2023;15(3):785. Essential reading on how WHO-HAEM5 separates classic FL from its related subtypes and from BCL2-negative pediatric-type FL.
- Alaggio R, Amador C, Anagnostopoulos I, et al. The 5th edition of the World Health Organization Classification of Haematolymphoid Tumours: Lymphoid Neoplasms. Leukemia. 2022;36(7):1720-1748.
- Wood BL, Arroz M, Barnett D, et al. 2006 Bethesda International Consensus recommendations on the immunophenotypic analysis of hematolymphoid neoplasia by flow cytometry: optimal reagents and reporting for the flow cytometric diagnosis of hematopoietic neoplasia. Cytometry B Clin Cytom. 2007;72 Suppl 1:S14-S22.
- Tsujimoto Y, Gorham J, Cossman J, Jaffe E, Croce CM. The t(14;18) chromosome translocations involved in B-cell neoplasms result from mistakes in VDJ joining. Science. 1985;229(4720):1390-1393.
- Davids MS, Roberts AW, Seymour JF, et al. Phase I first-in-human study of venetoclax in patients with relapsed or refractory non-Hodgkin lymphoma. J Clin Oncol. 2017;35(8):826-833.
- Hastings RJ, Moore S, Chia N, eds. ISCN 2024: An International System for Human Cytogenomic Nomenclature. Karger; 2024.
This article is educational and is not clinical guidance. Diagnostic and reporting practice follows your own laboratory's validated procedures and current professional standards.