This study will evaluate HIV-negative patients with unusually low levels of CD4+ T lymphocytes (a type of white blood cell) to learn more about the clinical symptoms, cause, immunology, and biology of this problem. CD4+ T lymphocytes play an important role in immune function, and low counts may leave people susceptible to unusual infections. CD4+ T cell deficiencies are most often associated with HIV infection. Patients 8 years of age and older with CD4+ T cell counts below 300 cells/mm3 who test negative for HIV infection by standard blood tests may be eligible for this study. Patients' family members and partners may also be enrolled to investigate the possible role of a genetic factor or exposure to some agent in this problem. Patients will be evaluated at the NIH Clinical Center at least once, and generally two or more times. The evaluations, which may be done on an inpatient or outpatient basis, will include some or all of the following tests and procedures: * Complete physical examination. * Medical history, including questions about sexual contacts, intravenous drug use, travel, blood transfusions, previous illnesses, including sexually transmitted diseases, and health of family members. * Urine test. * Blood tests for routine and research purposes, including tests for HIV, hepatitis, syphilis and other infections, evaluation of immune function, and culture for viruses in the HIV family. No more than 1 pint of blood will be drawn every 6 weeks. * Pregnancy test for women of childbearing potential. * Skin tests for tuberculosis and immune function. These tests involve injecting a small amount of the substance to be tested just under the skin and looking for a raised area 1 to 2 days later. * Apheresis. Whole blood is collected through an arm vein (similar to donating blood), and circulated through a cell separator machine, where it is spun to separate the components. The red cells are then returned to the patient either through the same needle or through a needle in the other arm, and the plasma and white cells are extracted for study. The procedure, which takes 1 to 2 hours, may be repeated up to 3 times. Family members will have 60 cc (4 tablespoons) of blood drawn to determine CD4+ T cell counts.
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