Lymphocytes normally comprise 20 to 40 percent of the total white blood cell count, according to MedlinePlus. A blood differential test determines the percentage of each type of white blood cell in the blood, including B cell and T cell lymphocytes.
High lymphocyte counts may be due to a chronic bacterial infection, infectious hepatitis, infectious mononucleosis, lymphocytic leukemia or multiple myeloma, states MedlinePlus. Certain viral infections such as mumps or measles also cause lymphocyte levels to rise. Chemotherapy, HIV infection, leukemia and radiation therapy or exposure decrease lymphocyte levels. Sepsis, a severe response to bacteria or germs, and steroid use also cause a reduction in lymphocyte counts.