Lymphocytosis

Lymphocytosis: > 33%

Etiology

• Acute viral infections (e.g., rubella, infectious mononucleosis, mumps)
• Chronic infections (e.g., tuberculosis, syphilis, toxoplasmosis)
• Neoplasia (e.g., Hodgkin lymphoma, non-Hodgkin lymphoma, CLL)

Tip

The lymphocyte count may be increased or decreased in lymphoma.

T cell differentiation

Cell	Induced by	Inhibited by	Secretes
Th1	IFN-γ, IL-12	IL-4, IL-10	IFN-γ, IL-2
Th2	IL-2, -4	IFN-γ	IL-4, -5, -6, -10, -13
Th17	IL-1, -6, TGF-β	IFN-γ, IL-4	IL-17, -21, and -22
Treg	TGF-β, IL-2	IL-6	TGF-β, IL-10, -35

Lymphatic system

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Lymph nodes

Function

• Nonspecific lymph filtration: macrophages within lymph node
• Storage and circulation of B cells and T cells
• Immune system activation: Antigen presentation induces differentiation and proliferation of B lymphocytes and activation of T lymphocytes.

Structure

Clinical significance

• Inflammatory or immune reactions (e.g., due to EBV infection) → reactive paracortical hyperplasia → clinically apparent lymphadenopathy
• Depletion of paracortical lymphocytes: DiGeorge syndrome