lymphocytosis

(>4000–5000/µL)

Infection

Usually viral; “atypical lymphocytes” with mononucleosis syndromes Other: pertussis, toxoplasmosis

Hypersensitivity

Drug-induced, serum sickness

Autoimmune

Rheumatoid arthritis (large granular lymphocytes), malignant thymoma

Neoplasm

Leukemia (eg, CLL, hairy cell, LGL), lymphoma (eg, mantle cell, folic.)