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Lymphocytes are small mononuclear cells without specific granules. These agranulocytes are motile cells that migrate to areas of inflammation in both early and late stages of the process. These cells are the source of serum immunoglobulins and cellular immune response and play an important role in immunologic reactions. All lymphocytes originate in the bone marrow. B lymphocytes mature in the bone marrow, and T lymphocytes mature in the thymus gland.


B cells control the antigen-antibody response that is specific to the offending antigen and are said to have "memory." Plasma cells (fully differentiated B cells) are similar in appearance to lymphocytes. They have abundant blue cytoplasm and an eccentric, round nucleus. Plasma cells are not normally present in blood. The T cells, the master immune cells, include helper/inducer T cells (CD4+) and suppressor/cytotoxic T cells (CD8+).


This test measures the number of lymphocytes in the peripheral blood. Lymphocytosis refers to an increase of peripheral blood lymphocytes, which for adults corresponds to more than 4,000 lymphocytes/microL in most clinical labs. Lymphocytopenia refers to a decrease of peripheral blood lymphocytes, which generally corresponds to fewer than 1,000 lymphocytes/microL in adults.1


Normal reference values in adults


* The normal range (two standard deviations above and below the mean) for the white blood cell (WBC) count in adults is 4,400 to 11,000 cells/microL in most clinical labs. Lymphocytes generally constitute 8% to 33% of WBCs in peripheral blood.


* Lymphocyte subsets-Normal proportions of T lymphocytes, B lymphocytes, and natural killer (NK) cells in the peripheral blood are:


* T cells (CD3+)-60% to 80%


* B cells (CD20+)-10% to 20%


* NK cells (CD56+)-5% to 10%.


* T lymphocyte subtypes-Normal proportions of T cell subtypes in peripheral blood are:


* helper/inducer T cells (CD4+)-60% to 70%


* suppressor/cytotoxic T cells (CD8+)-30% to 40%.1


Possible causes of lymphocytosis


* lymphatic leukemia (acute and chronic)


* lymphoma


* infectious lymphocytosis (occurs mainly in children)


* infectious mononucleosis


* other viral diseases


* viral infections of the upper respiratory tract (pneumonia)


* cytomegalovirus


* measles, mumps, chickenpox


* acute HIV infection


* infectious hepatitis (acute viral hepatitis)


* toxoplasmosis


* some bacterial diseases such as tuberculosis, brucellosis, and pertussis


* Crohn disease, ulcerative colitis (rare)


* serum sickness, drug hypersensitivity


* hypoadrenalism, Addison disease


* thyrotoxicosis (relative lymphocytosis)


* neutropenia with relative lymphocytosis.


Possible causes of lymphocytopenia


* chemotherapy, radiation treatment, immunosuppressive medications


* after administration of adrenocorticotropic hormone (ACTH) or cortisone (steroids); with ACTH-producing pituitary tumors


* increased loss through gastrointestinal tract owing to obstruction of lymphatic drainage (tumor, Whipple disease)


* aplastic anemia


* Hodgkin disease and other malignancies


* inherited immune disorders, AIDS, and AIDS immune dysfunction


* advanced tuberculosis ("miliary" tuberculosis), renal failure, systemic lupus erythematosus (SLE)


* severe debilitating illness of any kind


* heart failure.


Plasma cells (not normally present in blood) are increased in:


* plasma cell leukemia


* multiple myeloma


* Hodgkin disease


* chronic lymphatic leukemia


* liver, breast, or prostate cancer


* cirrhosis


* rheumatoid arthritis, SLE


* serum reaction


* some bacterial, viral, and parasitic infections.


Clinical alert:

A decreased lymphocyte count of less than 500/mm3 (<0.5 x 109/L) means that a patient is dangerously susceptible to infection, especially viral infections. Institute measures to protect the patient from infection.




Adapted from: Fischbach FT, Fischbach MA. A Manual of Laboratory and Diagnostic Tests. 10th ed. Philadelphia, PA: Wolters Kluwer; 2018.




1. Davids MS. Approach to the adult with lymphocytosis or lymphocytopenia. Up To Date. 2019. [Context Link]