Lymphocytes are a group of white blood cells involved in the operation of the immune system. They include natural killer cells, T cells, and B cells. They make up the majority of lymph tissue. They're created in the bone marrow.
A high lymphocyte count is a sign of a viral infection or, less commonly, leukemia. Low lymphocyte counts are almost always due to post-surgery infection, but can also be caused by HIV, which attacks T cells.