Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
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. It is sometimes erroneously called a "Downy cell".
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
- Treponema pallidum (Syphilis)
- Streptococcus agalactiae (Group B Streptococci),
- Hepatitis C
- ↑ Cabot, Richard C.; Scully, Robert E.; Mark, Eugene J.; McNeely, William F.; McNeely, Betty U.; Rosenfield, Cathy G.; Kaplan, Mark A. (June 1994). "Case records of the Massachusetts General Hospital. Weekly clinicopathological exercises. Case 24-1994. A two-year-old boy with thrombocytopenia, leukocytosis, and hepatosplenomegaly". N. Engl. J. Med. 330 (24): 1739–46. doi:10.1056/NEJM199406163302408. PMID 8190136.
- ↑ Downey H, McKinlay CA. Acute Lymphadenosis Compared with Acute Lymphatic Leukemia. Arch Intern Med. 1923;32:82-112
- ↑ "MedlinePlus Medical Encyclopedia: Mononucleosis, photomicrograph of cells". Retrieved 2009-03-07.
- ↑ Peters CJ, Khan AS (2002). "Hantavirus pulmonary syndrome: the new American hemorrhagic fever". Clin Infect Dis. 34 (9): 1224&ndash, 31. doi:10.1086/339864. PMID 11941549.
- 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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