Lymphocytosis can be either primary or secondary.
Primary lymphocytosis, often called lymphoproliferative disease, is caused by dysregulation in the production of lymphocytes. The reactive, or secondary, lymphocytoses are conditions that involve absolute increases in lymphocytes caused by physiologic or pathophysiological responses to infection, inflammation, toxins, cytokines, or unknown agents. Persistent lymphocytosis is seen in patients with autoimmune disorders, sarcoidosis, hyposplenism, and cancer and in those who are chronic cigarette smokers. Because 80 percent of adult lymphocytes are T cells, most cases of lymphocytopenia are caused by a reduction in the T cell count. The mechanisms of lymphocytopenia are often unknown, and the causes are usually differentiated as either inherited or acquired.