What are the causes of increased lymphocytes in leukocytosis?

Updated: May 19, 2020
  • Author: Susumu Inoue, MD; Chief Editor: Jennifer Reikes Willert, MD  more...
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Lymphocytosis conventionally refers to a lymphocyte count greater than 4 X 109/L (4000/µL); however, a lymphocyte count that exceeds this is physiologically present in infants and young children. The upper normal limit of lymphocyte count in this age group has not been well defined in a healthy population.

Marked lymphocytosis is observed in individuals infected with pertussis (total leukocyte count of 40-50 X 109/L, or X 40-50 X 103/µL). An exceedingly high lymphocyte count such as 100 X 109/L indicates a poor prognosis.

Viral infection generally causes lymphocytosis (relative or absolute) with or without neutropenia. Typical examples include infectious mononucleosis or cytomegalovirus infection, respiratory syncytial virus infections, and infectious hepatitis. On the other hand, some viral infection results in remarkable leukemoid reaction with a shift to left. An example is the Hantavirus pulmonary syndrome. [30] The highest WBC count during the 1993 outbreak was reported to be 65,000/μL with the shift to left. The author has seen neutrophilia and leukocytosis in the early phase of Epstein-Barr virus infection in children. Reports have found that in (mostly adult) patients with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus that causes COVID-19, those suffering from respiratory failure presented with profound lymphopenia. [39]

Chronic lymphocytic leukemia that is routinely characterized by mature lymphocytosis is extremely rare in children and is usually not considered in the differential diagnosis of lymphocytosis.

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