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Lymphocytic pleocytosis is an abnormal increase in the amount of lymphocytes in the cerebrospinal fluid. It is usually considered to be a sign of infection or inflammation within the nervous system, and is encountered in a number of neurological diseases, such as pseudomigraine, Susac's syndrome, and encephalitis. While lymphocytes make up roughly a quarter of all white blood cells in the body, they are generally rare in the CSF. Under normal conditions, there are usually less than 5 white blood cells per µL of CSF. In a pleocytic setting, the number of lymphocytes can jump to more than 1,000 cells per µL. Increases in lymphocyte count are often accompanied by an increase in cerebrospinal protein concentrations in addition to pleocytosis of other types of white blood cells.