A higher amount of reactive lymphocytes is seen in patients with viral illnesses such as infectious mononucleosis and viral hepatitis and infections with cytomegalovirus, HIV and rubella. Reactive lymphocytes with convoluted nuclei can been seen in patients with T-cell lymphomas or Sezary syndrome, states Edward C. Lynch in "Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd edition."
Reactive lymphocytes are also called atypical lymphocytes, or Downey cells. They are larger than normal lymphocytes and contain a greater amount of cytoplasm, according to the American Association for Clinical Chemistry.
A common cause of elevated reactive lymphocytes is infectious mononucleosis. This disease most commonly affects people between the ages of 10 and 30. Infectious mononucleosis is suspected when patients present with clinical signs of a sore throat, fatigue, petechiae of the palate and an enlargement of the lymph nodes together with a 20 percent increase in reactive lymphocytes on a peripheral smear. In fact, a 20 percent increase in atypical lymphocytes alone can suggest the presence of this infection, explains Dr. Mark Ebell for the American Academy of Family Physicians.
Hoagland’s criteria are most commonly used to make an accurate diagnosis of infectious mononucleosis. The criteria require patients to present with a fever, pharyngitis and adenopathy, concurrently with at least 10 percent atypical lymphocytes and 50 percent lymphocytosis. A positive serologic test for the infection is also necessary, notes Dr. Ebell.