Step 1 topics → Hematology and Oncology → Chronic Lymphocytic Leukemia (CLL) / SLL

Chronic Lymphocytic Leukemia (CLL) / SLL

USMLE Step 1 trap: Confuses CLL immunophenotype (CD5+/CD23+) with follicular lymphoma (CD10+/CD5−). CLL/SLL cells co-express CD5 and CD23 (with dim surface Ig), whereas follicular lymphoma expresses CD10 but not CD5.

CLL and its tissue counterpart SLL are the same disease — malignant CD5+ B cells accumulating in blood/marrow (CLL) or lymph nodes (SLL). It's the most common leukemia in adults in the Western world, and USMLE Step 1 tests it hard. The most common immunophenotyping error: assigning CD10 to CLL. CD10 is a follicular lymphoma marker — CLL is CD10-negative and instead co-expresses CD5 and CD23, an unusual combination that specifically flags CLL among B-cell malignancies. Getting that distinction wrong means misidentifying the disease on any flow cytometry question. The classic vignette is an older adult with asymptomatic lymphocytosis, but the exam will push further into smudge cells, flow cytometry, and Richter transformation recognition.

What makes CLL tricky is that it requires you to hold multiple marker sets in your head simultaneously. Students consistently confuse the CLL immunophenotype (CD5+, CD23+, dim surface Ig) with follicular lymphoma (CD10+, CD5−, bcl-2+). The CD5 co-expression with a B-cell marker is the key distinguishing feature, and Step 1 will absolutely test whether you know that CLL is one of the few B-cell malignancies that co-expresses CD5.

The other high-yield trap is understanding WHY CLL patients get infections. It's not neutropenia — it's hypogammaglobulinemia from dysfunctional B cells (immune paresis), which leaves patients exposed to encapsulated organisms like S. pneumoniae and H. influenzae. USMLE Step 1 rewards students who can explain the mechanism, not just recall the association. Know the complications — autoimmune hemolytic anemia, immune paresis, and Richter transformation — because those are the clinical turning points the exam loves to vignette.

From real student decks
70%have cards covering this topic
49%have mature cards
1.0median lapses in 14 days

Common misconceptions

Common mistake
Wrong: CLL cells express CD10 and CD23 like follicular lymphoma.
Right: CLL/SLL cells co-express CD5 and CD23 (with dim surface Ig), whereas follicular lymphoma expresses CD10 but not CD5.
CD10 is a marker of follicular lymphoma and other germinal center-derived B-cell malignancies — it is not expressed in CLL. CLL is unique among B-cell malignancies for co-expressing CD5, which is normally a T-cell marker. The full CLL phenotype is CD5+/CD19+/CD23+/dim surface Ig, while follicular lymphoma is CD10+/CD5−/bcl-2+. When you see CD5 on a B cell, think CLL or mantle cell lymphoma (mantle cell adds cyclin D1 and lacks CD23).
Common mistake
Wrong: Richter transformation converts CLL into an acute leukemia.
Right: Richter transformation is the conversion of CLL into an aggressive large B-cell lymphoma (most commonly DLBCL), not an acute leukemia.
Blast crisis is what happens in CML — the chronic phase accelerates into an acute leukemia-like state with immature blasts. Richter transformation is completely different: it's the sudden conversion of CLL into a high-grade, aggressive large B-cell lymphoma, most commonly diffuse large B-cell lymphoma (DLBCL). Clinically, a CLL patient with Richter transformation will have rapid lymph node enlargement, systemic B symptoms (fevers, night sweats, weight loss), and a dramatically worsened prognosis. The cells are large and aggressive, not blastic.
Common mistake
Wrong: CLL patients are most susceptible to infections because of neutropenia.
Right: CLL causes hypogammaglobulinemia (immune paresis) from dysfunctional B cells, making patients susceptible to encapsulated bacterial infections despite often-normal neutrophil counts.
CLL fills the bone marrow and peripheral blood with dysfunctional, non-immunocompetent B cells that cannot produce normal antibodies. This crowds out normal B-cell clones and leads to hypogammaglobulinemia — a state called immune paresis. Neutrophil production is largely intact in CLL (unlike AML), so neutropenic infections are not the primary concern. Instead, the lack of opsonizing antibodies makes patients susceptible to encapsulated bacteria (S. pneumoniae, H. influenzae, N. meningitidis) that require antibody-mediated opsonization for clearance.
Free Deck audit

See if your Anki deck covers this topic.

Upload your deck →
Guided session

Stuck on this? An AI tutor that probes your understanding.

Start a session →

What the exam tests

  1. Recognize the typical CLL patient (elderly adult, often asymptomatic) and explain why these patients are prone to infections with encapsulated bacteria specifically — not due to low neutrophil counts but due to dysfunctional B cells causing hypogammaglobulinemia.
  2. Identify CLL on a peripheral blood smear (smudge cells from fragile lymphocytes) and correctly assign the flow cytometry immunophenotype: CD5+, CD19+, CD20 (dim), CD23+, with dim surface immunoglobulin — and distinguish this from follicular lymphoma markers.
  3. Recognize and explain the major complications of CLL: Richter transformation (conversion to DLBCL, not blast crisis), autoimmune cytopenias (especially warm AIHA via autoantibody production), and immune paresis (hypogammaglobulinemia leading to recurrent sinopulmonary infections).

Can you avoid these mistakes?

A 68-year-old man has an incidental WBC of 42,000 with 85% small mature lymphocytes. Peripheral smear shows smudge cells. Flow cytometry shows CD5+, CD19+, CD23+, dim surface IgM. What is the diagnosis, and how does this immunophenotype differ from follicular lymphoma?
Your CLL patient develops worsening fatigue, a new positive direct Coombs test, and a hemoglobin of 7.5 g/dL. His neutrophil count is normal. What is the mechanism of his anemia, and separately, why is he at risk for pneumococcal pneumonia despite normal neutrophils?
A known CLL patient presents with rapidly enlarging cervical lymphadenopathy, fever, and 15-lb weight loss over 6 weeks. LDH is markedly elevated. Biopsy shows large atypical lymphoid cells. What transformation has occurred, and what is it most commonly transforming into?
Flow cytometry on a lymph node biopsy shows CD5+, CD19+, CD10−, CD23−, cyclin D1+. Is this CLL/SLL or a different diagnosis, and what chromosomal translocation is classically associated with this alternative diagnosis?

Related topics

Leukemia vs Lymphoma Framework Hematopoiesis and Lineage Differentiation RBC Physiology and Lifespan WBC Differential and Function Platelet Physiology and Primary Hemostasis

See how your Anki deck covers this topic.

Upload your deck for a free audit →