Lymphocytosis, an elevated lymphocyte count in the blood, can arise from various underlying causes, broadly categorized as reactive (due to an immune response) or due to monoclonal expansion (proliferation of a single abnormal lymphocyte clone).
Reactive Lymphocytosis Causes
Reactive lymphocytosis occurs when the body increases lymphocyte production in response to an external stimulus. Common causes include:
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Infections: Viral, bacterial, and parasitic infections can all trigger lymphocytosis. Examples include:
- EBV (Epstein-Barr Virus), causing mononucleosis
- CMV (Cytomegalovirus)
- Pertussis (whooping cough)
- Cat-scratch disease
- HIV (Human Immunodeficiency Virus)
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Drug Reactions: Certain medications can induce lymphocytosis as part of a drug reaction with eosinophilia and systemic symptoms (DRESS) syndrome.
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Stress: Physiological stress can temporarily elevate lymphocyte counts.
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Asplenia: Absence of the spleen, either congenital or acquired (surgical removal), can lead to a chronic, mild lymphocytosis.
Lymphocytosis Secondary to Monoclonal Expansion Causes
This type of lymphocytosis involves the uncontrolled proliferation of a single abnormal lymphocyte clone. Common causes include:
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CLL/SLL: Chronic Lymphocytic Leukemia/Small Lymphocytic Lymphoma
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MBL: Monoclonal B-cell Lymphocytosis (ALC under 5000 cells/microL). Note: When the absolute lymphocyte count (ALC) is below 5000 cells/microL, it is classified as MBL.
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NHL: Non-Hodgkin Lymphomas, including:
- MZL (Marginal Zone Lymphoma)
- FL (Follicular Lymphoma)
- MCL (Mantle Cell Lymphoma)
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T-LGL: T-cell Large Granular Lymphocytic Leukemia
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HCL: Hairy Cell Leukemia
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Sézary Syndrome: A cutaneous T-cell lymphoma
