• Normal infants and young children <5 have a higher proportion and concentration of lymphocytes than adults.

• Rare in acute bacterial infection except in pertussis (may be >50 x 109/L).

• Acute infectious lymphocytosis also seen in children, usually associated with transient lymphocytosis and a mild constitutional reaction.

• Characteristic of infectious mononucleosis but these lymphocytes are often large and atypical and the diagnosis may be confirmed with a heterophil agglutination test.

• Similar atypical cells may be seen in patients with CMV and hepatitis A infection.

• Chronic infection with brucellosis, tuberculosis, secondary syphilis and congenital syphilis may cause lymphocytosis.

• Lymphocytosis is characteristic of CLL, ALL and occasionally NHL.

Where primary haematological cause suspected, immunophenotypic analysis of the peripheral blood lymphocytes will often confirm or exclude a neoplastic diagnosis. BM examination is indicated if neoplasia is strongly suspected and in any patient with concomitant neutropenia, anaemia or thrombocytopenia.

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