HealthCentral explains that lymphocytosis, or the presence of a high number of lymphocytes, can be caused by many different disorders and diseases, including mononucleosis, toxoplasmosis, AIDS and tuberculosis. In addition to these serious diseases, lymphocytosis can merely signal that a person is suffering from an upper respiratory infection or the after-effects of a traumatic injury. The condition can also indicate acute or chronic lymphocytic leukemia or lymphoma.
The Mayo Clinic asserts that the exact thresholds at which a high number of lymphocytes in a blood sample is considered to lymphocytosis varies from one lab to the next; however, adults with more than 3,000 lymphocytes per milliliter of blood are generally considered to suffer from lymphocytosis. Children have much higher lymphocyte counts than adults, and the count tends to vary with age. In some cases, children must have more than 9,000 lymphocytes per milliliter of blood to be considered to suffer from lymphocytosis.
HealthCentral states that some scientists believe that lymphocytes are responsible for immunological memory. This capability means that once a person is exposed to a pathogen, his body is able to mount a quicker defense when exposed to the same pathogen in the future. Lymphocytes are produced by the same stem cells that produce red blood cells.