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 of cellular immune response and play an important role in immunologic reactions. All lymphocytes are manufactured 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 is said to have “memory.” The T cells, the master immune cells, include CD4+ helper T cells, killer cells, cytotoxic cells, and CD8+ suppressor T cells.

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.

This test measures the number of lymphocytes in the peripheral blood. Lymphocytosis is present in various diseases and is especially prominent in viral disorders. Lymphocytes and their derivatives, the plasma cells, operate in the immune defenses of the body.

Lymphocytes: 25%–40% of total leukocyte count (relative value) or 1000–4000 cells/mm³ or 1.0–4.0 × 10⁹/L

Plasma cells: 0% or none

CD4 count: total WBC count × lymphocytes (%) × lymphocytes (%) stained with CD4

CD4/CD8 ratio: >1.0

1. Obtain 5 mL of whole blood in a lavender-topped tube (with EDTA). Label the specimen with the patient’s name, date and time of collection, and test(s) ordered.

2. Count lymphocytes as part of the differential count.

1. Lymphocytosis: >4000/mm³ or >4.0 × 10⁹/L in adults; >7200/mm³ or >7.2 × 10⁹ in children; and >9000/mm³ or >9.0 × 10⁹/L in infants occurs in:

   1. Lymphatic leukemia (acute and chronic) lymphoma

   2. Infectious lymphocytosis (occurs mainly in children)

   3. Infectious mononucleosis:

      1. Caused by Epstein–Barr virus

      2. Most common in adolescents and young adults

      3. Characterized by atypical lymphocytes (Downey cells) that are large and deeply indented, with deep blue (basophilic) cytoplasm

      4. Differential diagnosis—positive heterophil test

   4. Other viral diseases:

      1. Viral infections of the upper respiratory tract (pneumonia)

      2. Cytomegalovirus

      3. Measles, mumps, chickenpox

      4. Acute HIV infection

      5. Infectious hepatitis (acute viral hepatitis)

      6. Toxoplasmosis

   5. Some bacterial diseases such as tuberculosis, brucellosis (undulant fever), and pertussis

   6. Crohn disease, ulcerative colitis (rare)

   7. Serum sickness, drug hypersensitivity

   8. Hypoadrenalism, Addison disease

   9. Thyrotoxicosis (relative lymphocytosis)

   10. Neutropenia with relative lymphocytosis

2. Lymphopenia: <1000 cells/mm³ or <1.0 × 10⁹/L in adults; <2500 cells/mm³ or <2.5 × 10⁹/L in children occurs in:

   1. Chemotherapy, radiation treatment, immunosuppressive medications

   2. After administration of ACTH or cortisone (steroids); with ACTH-producing pituitary tumors

   3. Increased loss through gastrointestinal tract owing to obstruction of lymphatic drainage (e.g., tumor, Whipple disease, intestinal lymphectasia)

   4. Aplastic anemia

   5. Hodgkin disease and other malignancies

   6. Inherited immune disorders, acquired immune deficiency syndrome (AIDS), also known as late-stage HIV, and AIDS immune dysfunction

   7. Advanced tuberculosis (“miliary” tuberculosis), chronic kidney disease (CKD), SLE

   8. Severe debilitating illness of any kind

   9. Heart failure

3. CD4 count: The number of CD4+ lymphocytes is equal to the absolute number of lymphocytes (total WBC count × differential [%] of lymphocytes) times the percentage of lymphocytes staining positively for CD4. A severely depressed CD4 count is the single best indicator of imminent opportunistic infection.

   1. Decreased CD4 lymphocytes

      1. Immune dysfunction, especially AIDS. For the CD4, the diagnosis of AIDS is made for counts less than 200. There is a 1:3 ratio between Hb and Hct.

      2. Acute minor viral infections

   2. Increased CD4 lymphocytes

      1. Therapeutic effect of drugs

      2. Diurnal variation: Peak evening values may be two times morning values.

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

   1. Plasma cell leukemia

   2. Multiple myeloma

   3. Hodgkin disease

   4. Chronic lymphatic leukemia

   5. Cancer of liver, breast, prostate

   6. Cirrhosis

   7. RA, SLE

   8. Serum reaction

   9. Some bacterial, viral, and parasitic infections