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Lymphocytes (Absolute Lymphocyte Count)

SI UNITS (recommended)

CONVENTIONAL UNITS

(Core Immune Cell Count — Key Marker in Viral Infections, Immunodeficiency, Autoimmunity & Hematologic Disorders)

Synonyms

  • Absolute lymphocyte count (ALC)
  • Lymphocyte count
  • Lymphs
  • Lymphocyte number
  • Total lymphocytes

Units of Measurement

  • 10⁹/L
  • G/L
  • Gpt/L
  • cells/L
  • 10³/µL
  • 1000/µL
  • 10³/mm³
  • 1000/mm³
  • K/µL
  • K/mm³
  • cells/µL
  • cells/mm³

Key Unit Equivalences

All the following are identical:

1 × 10⁹/L = 1 G/L = 1 Gpt/L = 1000 cells/µL = 1 K/µL = 1 × 10³/µL = 1000/mm³

Conversions

  • 1 × 10⁹/L = 1000 cells/µL
  • 1 K/µL = 1 × 10⁹/L

Description

Lymphocytes are a major class of white blood cells involved in adaptive and innate immunity.
Types include:

  • T lymphocytes (T cells) → cellular immunity
  • B lymphocytes (B cells) → antibody production
  • NK cells → innate cytotoxic immunity

Absolute lymphocyte count (ALC) measures the true number of lymphocytes in blood, offering crucial insight into immune system status.

Physiological Role

1. T Lymphocytes

  • CD4⁺ helper T cells → coordinate immune responses
  • CD8⁺ cytotoxic T cells → kill infected/tumor cells

2. B Lymphocytes

  • Produce immunoglobulins (IgG, IgA, IgM, etc.)
  • Memory B cells maintain long-term immunity

3. Natural Killer (NK) Cells

  • Kill virus-infected cells & cancer cells

4. Immune Memory

  • Vaccine responsiveness
  • Long-term protection

Clinical Significance

HIGH Lymphocytes

1. Viral Infections

  • EBV / infectious mononucleosis
  • CMV
  • Viral hepatitis
  • HIV (early)
  • Influenza
  • Adenovirus

2. Chronic Lymphocytic Leukemia

  • Marked persistent lymphocytosis
  • Often ALC > 5.0 × 10⁹/L

3. Post-infection Rebound

Common in recovering children.

4. Smoking

Mild persistent lymphocytosis.

5. Thyrotoxicosis

6. Autoimmune Conditions

  • ITP
  • RA
  • Sjögren syndrome

7. Drug Effects

  • Some antipsychotics
  • Corticosteroid withdrawal

LOW Lymphocytes

(Clinically critical)

1. Acute Infections

Especially:

  • Sepsis
  • Severe viral infections
  • COVID-19 (hallmark finding)

2. HIV Infection

CD4 depletion causes profound lymphopenia.

3. Primary Immunodeficiencies

  • SCID
  • DiGeorge syndrome
  • Wiskott–Aldrich syndrome

4. Autoimmune Diseases

  • SLE
  • Sarcoidosis

5. Bone Marrow Disorders

  • Aplastic anemia
  • Chemotherapy / radiation
  • Leukemia / lymphoma

6. Corticosteroids

Cause significant lymphocyte redistribution & suppression.

7. Protein-Energy Malnutrition

8. Stress Response

Acute stress → transient lymphopenia.

Reference Intervals

(Tietz 8E + Mayo + ARUP + WHO)

Adults

  • 1.0 – 3.5 × 10⁹/L
     (= 1000 – 3500 cells/µL)

Children

Higher due to active immune development:

AgeNormal ALC
Newborn2.0 – 11.0 ×10⁹/L
1–2 years3.0 – 9.5 ×10⁹/L
2–6 years2.0 – 8.0 ×10⁹/L
>6 years1.0 – 5.0 ×10⁹/L

Critical Levels

  • ALC < 0.5 × 10⁹/L → high infection risk
  • ALC > 5.0 × 10⁹/L → consider CLL or chronic viral infection

Diagnostic Uses

1. Evaluate Infection

  • Viral infections → lymphocytosis
  • Severe bacterial infection → lymphopenia

2. Screen for Immunodeficiency

Persistently low ALC suggests:

  • HIV
  • SCID
  • Other T/B cell defects

3. Hematologic Malignancies

CLL, lymphoma → high persistent ALC.

4. Autoimmune Disorders

ALC helps track activity (e.g., SLE).

5. Treatment Monitoring

Chemotherapy and immunosuppressants suppress lymphocytes.

6. COVID-19 Severity Marker

Low ALC strongly predicts:

  • ICU admission
  • Mortality

Analytical Notes

  • Derived from automated CBC analyzers
  • Verified with peripheral smear if abnormal
  • Pseudolymphocytosis may occur with platelet clumping
  • EDTA samples required
  • Stress, exercise, and diurnal variation can alter counts

Clinical Pearls

  • In CLL, ALC >5 ×10⁹/L for >3 months is diagnostic (with smear/immunophenotype).
  • ALC < 0.5 ×10⁹/L → serious infection risk.
  • ALC is a simple but powerful marker in sepsis and COVID-19.
  • Children naturally have higher lymphocyte counts than adults.
  • Corticosteroids cause lymphocyte redistribution, not destruction.

Interesting Fact

In infants, lymphocytes are the predominant white cell type, unlike in adults where neutrophils dominate - this shift is part of the “leukocyte differential crossover” that occurs around age 5.

References

  1. Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Hematology
  2. WHO Hematology Reference Ranges
  3. Mayo Clinic Laboratories - Lymphocytes
  4. ARUP Consult - CBC Interpretation
  5. MedlinePlus / NIH - Lymphocyte Count

Last updated: January 26, 2026

Reviewed by : Medical Review Board

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