Lymphocytes
Lymphocytes· also: Lymphocytes
Clinical Overview
Lymphocytes are white blood cells of the adaptive immune system, consisting of B cells (antibody production), T cells (cellular immunity), and NK cells (natural killer cells). They are the second most abundant WBC type in adults and the most abundant in children. Lymphocytes are the predominant cell in viral infections.
Why This Test Matters
Lymphocytosis is a hallmark of viral infections, particularly infectious mononucleosis (EBV), CMV, and pertussis. Chronic lymphocytic leukemia (CLL) — the most common adult leukemia — presents with markedly elevated lymphocyte counts. Lymphopenia is a key feature of HIV infection (CD4 T cell depletion), severe sepsis, and autoimmune diseases like lupus.
Reference RangesWHO/IFCC standards
| Age Group | Reference Range | Unit | Notes |
|---|---|---|---|
| Adults (18–64) | 1 – 4.8 | ×10³/µL | — |
Also reported in: %.
What Causes Abnormal Results?
High Lymphocytes Causes
- Viral infections: EBV (mononucleosis), CMV, hepatitis, pertussis, COVID-19
- Chronic lymphocytic leukemia (CLL)
- Acute lymphoblastic leukemia (ALL) in children
- Thyrotoxicosis
- Post-vaccination response
Low Lymphocytes Causes
- HIV/AIDS (CD4 T cell destruction)
- Corticosteroid therapy
- Chemotherapy and radiation
- Autoimmune diseases (lupus)
- Severe sepsis
- Congenital immunodeficiency syndromes
Signs & Symptoms to Watch For
How to Prepare for This Test
No special preparation. Part of the CBC differential.
Factors That Can Affect Results
- Corticosteroids (significantly lower lymphocyte count)
- Age (children have normally higher lymphocyte percentages)
- Viral illness timing (may be highest in acute phase)
Related Topics
Frequently Asked Questions
What is a reactive lymphocyte?
Reactive lymphocytes (also called atypical lymphocytes or Downey cells) are enlarged, activated T cells seen in response to viral infections, especially EBV (infectious mononucleosis). They appear on a blood smear and are reported by the laboratory. More than 10% reactive lymphocytes in the differential is characteristic of mononucleosis.
When is a high lymphocyte count concerning for leukemia?
A persistently elevated absolute lymphocyte count above 5,000/µL, especially in adults over 50, should raise concern for chronic lymphocytic leukemia (CLL). The diagnosis requires flow cytometry to identify the characteristic CLL immunophenotype (CD5+/CD19+/CD23+ B cells). CLL is often indolent and may not require immediate treatment.