text: what is relative lymphocytosis?   lymphocytosis is an increase in the number of lymphocytes in the blood. lymphocytosis conventionally refers to a lymphocyte count greater than 4000 per microliter, however the number may be higher in infants and children. it is common in acute viral infections, including glandular fever, but it may also be seen with intracellular bacterial infections, such as in tuberculosis. the malignant blood disease chronic lymphocytic leukaemia can cause a massive lymphocytosis, as can other leukaemias. lymphocytosis, preceded by leukopenia, is also seen in the icteric phase of a viral hepatitis infection.