Lymphopenia in Children

Lymphocytes account for about 30% of the circulating WBCs in a newborn. The proportion of lymphocytes then increases rapidly within the 1st mo, reaching an average of 60% by 2 yr of age. The normal lymphocyte count in children <2 yr of age is 3,000–9,500/?L and in adults is 1,000–4,800/?L. At 6 yr of age, the lower limit of normal is 1,500/?L.

Almost 65% of blood T lymphocytes are CD4 (helper) T lymphocytes. Most patients with lymphocytopenia have a reduction in the absolute number of T lymphocytes, particularly in the number of CD4 T lymphocytes. The average number of CD4 T lymphocytes in adult blood is 1,100/?L (range, 300–1,300/?L), and the average number of CD8 (suppressor) T lymphocytes is 600/?L (range, 100–900/?L), with the normal CD4:CD8 ratio of 1.8–2.0.

Lymphocytopenia by itself usually causes no symptoms and is often detected in the evaluation of other illnesses, particularly recurrent viral, fungal, and parasitic infections. Lymphocyte subpopulations can be measured by multiparameter flow cytometry, which uses the pattern of antigen expression to classify and characterize these cells.

INHERITED CAUSES OF LYMPHOCYTOPENIA
Inherited immunodeficiency disorders may have a quantitative or qualitative stem cell abnormality resulting in ineffective lymphocytopoiesis. Other disorders such as Wiskott-Aldrich syndrome may have associated lymphocytopenia arising from accelerated destruction of T cells. A similar mechanism is present in patients with adenosine deaminase deficiency and purine nucleoside phosphorylase deficiency.

ACQUIRED LYMPHOCYTOPENIA. Acquired lymphocytopenia is the result of depletion of blood lymphocytes that is not secondary to inherited diseases. AIDS is the most common infectious disease associated with lymphocytopenia, which results from destruction of CD4 T cells infected with HIV-1 or HIV-2. Other viral and bacterial diseases may be associated with lymphocytopenia. In some instances of acute viremia with other viral infections, lymphocytes may undergo accelerated destruction from intracellular viral replication, become trapped in the spleen or nodes, or migrate to the respiratory tract.

Iatrogenic lymphocytopenia is usually caused by cytotoxic chemotherapy, radiation therapy, and long-term administration of antilymphocyte globulin. Long-term treatment of psoriasis with psoralen and ultraviolet irradiation may destroy T lymphocytes. Corticosteroids can cause lymphopenia through increased cell destruction. Systemic autoimmune diseases such as systemic lupus erythematosus are associated with lymphocytopenia. Other conditions such as protein-losing enteropathy and aberrant or surgical drainage of the thoracic duct are associated with lymphocyte depletion, leading to lymphocytopenia.