Reactive lymphocytes are lymphocytes that become large as a result of antigen stimulation. Typically they can be more than 30 µm in diameter with varying size and shape.

The nucleus of a reactive lymphocyte can be round, elliptic, indented, cleft or folded. The cytoplasm is often abundant and can be basophilic. Vacuoles and/or azurophilic granules are also sometimes present. Most often the cytoplasm is gray, pale blue or deep blue in colour.

The distinctive cell associated with EBV or CMV is known as a “Downey cell”, after Hal Downey, who contributed to the characterization of it in 1923.^[1][2] It is sometimes erroneously called a “Downy cell”.^[3]

Reactive lymphocytes are usually associated with viral illnesses, however, they can also be present as a result of drug reactions (such as phenytoin), immunisations, radiation, hormonal causes (such as stress and Addison’s disease) as well as some auto-immune disorders (such as rheumatoid arthritis).

Some pathogen-related causes include:

• Epstein-Barr virus
• Cytomegalovirus
• Treponema pallidum (Syphilis)
• Streptococcus agalactiae (Group B Streptococci),
• Hepatitis C
• Hantavirus^[4]

• Review Article: The Atypical Lymphocyte– International Pediatrics Volume 18, No. 1; Michael W. Simon.
• Educational Commentary: Blood Cell Identification – American Society for Clinical Pathology article.

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