Lymphocytes are a subset of white blood cells (WBC) that form an integral part of the immune system. They facilitate the body's humoral and cellular immunity against foreign proteins and pathogens. An increase in circulating lymphocytes (lymphocytosis) can be seen following infections such as infectious mononucleosis and pertussis, or in lymphoproliferative disorders such as acute and chronic lymphocytic leukemia. A similar array of disorders can cause a decrease in lymphocyte counts (lymphocytopenia), although immunodeficiency states must also be considered, especially in infants and young children.
The various causes of lymphocytosis and lymphocytopenia will be reviewed here. Information on specific malignant conditions associated with abnormal lymphocyte counts (eg, chronic lymphocytic leukemia, large granular lymphocyte leukemia, acute lymphoblastic leukemia) is presented separately.
●(See "Clinical presentation, pathologic features, diagnosis, and differential diagnosis of chronic lymphocytic leukemia".)
●(See "Clinical manifestations, pathologic features, and diagnosis of T cell large granular lymphocyte leukemia".)
●(See "Clinical manifestations, pathologic features, and diagnosis of precursor B cell acute lymphoblastic leukemia/lymphoma".)