Granulocytopenia may result from diminished production of granulocytes in bone marrow, increased peripheral destruction of granulocytes, or greater use of granulocytes. Diminished production of granulocytes in bone marrow generally stems from radiation therapy or drug therapy. Drug-induced granulocytopenia usually develops slowly and typically correlates with the dosage and duration of therapy. Granulocyte production also decreases in such conditions as aplastic anemia and malignant bone marrow diseases and in some hereditary disorders.

Infections such as mononucleosis may cause granulocytopenia because of an increased use of granulocytes.

Similarly, lymphocytopenia may result from decreased production, increased destruction, or loss of lymphocytes. Decreased lymphocyte production may result from a genetic or thymic abnormality or from an immunodeficiency disorder, such as thymic dysplasia or ataxia-telangiectasia. Increased lymphocyte destruction may be caused by radiation therapy, chemotherapy, or human immunodeficiency virus infection. Loss of lymphocytes may follow postoperative thoracic duct drainage, intestinal lymphangiectasia, or impaired intestinal lymphatic drainage (as in Whipple’s disease). Lymphocyte depletion can also result from elevated plasma corticoid levels (due to stress, corticotropin or steroid therapy, or heart failure). Other disorders associated with lymphocyte depletion include Hodgkin’s disease, leukemia, aplastic anemia, sarcoidosis, myasthenia gravis, systemic lupus erythematosus, protein-calorie malnutrition, renal failure, terminal cancer, tuberculosis and, in infants, severe combined immunodeficiency (SCID) disease.

Localized infection can quickly become systemic (as in bacteremia) or spread throughout an organ (as in pneumonia).