Background: * Idiopathic CD4+ lymphocytopenia (ICL) is a condition in which there is a decreased level of CD4+ lymphocytes (a type of white blood cell), which can lead to opportunistic infections or autoimmune disorders and diseases. Objectives: * To characterize the natural history with regard to CD4+ T cell count and onset of infection, malignancy, and autoimmunity. * To describe the immunological status of patients affected by ICL while providing the best possible standard therapy to eradicate opportunistic infections. * To establish the timeline of CD4 lymphocytopenia, with particular focus on defining subgroups of patients according to the decline, stabilization, or rise of CD4+ T cell counts over time. * To characterize the opportunistic infections that occur in ICL patients at microbiologic and molecular levels. * To characterize the immunophenotype and possible genetic immunodeficiency causes of ICL. * To determine whether measurable immunologic parameters correlate with the development of opportunistic infections or other comorbidities such as lymphoma in patients with ICL. * To determine whether there is any association between ICL and autoimmunity. * To determine CD4+ T cell turnover, survival, functionality, and cytokine responsiveness in ICL patients. Eligibility: * Patients 2 years of age and older with an absolute CD4 count less than 300 in children 6 years or older and adults or less than 20% of T cells in children younger than 6 on two occasions at least 6 weeks apart. * Patients with negative results of HIV testing by ELISA, Western Blot, and viral load. * Patients must not have underlying immunodeficiency conditions, be receiving cytotoxic chemotherapy (anti-cancer drugs that kill cells), or have cancer. Design: * At the initial visit to the National Institutes of Health, the following evaluations will be conducted: * Personal and family medical histories. * Physical examination, including rheumatology evaluation and other consultations as medically indicated (e.g., dermatology, pulmonology, ophthalmology, imaging studies). * Blood samples for analysis of red and white blood cell counts, liver function, immune hormones, and antibody and autoantibody levels, white blood cell growth and function, and DNA. * Urinalysis and urine pregnancy testing for female patients of childbearing age. * Evaluation and treatment of active infections as medically indicated, including biopsies, buccal swabs, pulmonary function tests, and imaging studies. * Follow-up visits will take place approximately every 12 months or more frequently if indicated, and will continue for a minimum of 4 years and a maximum of 10 years. * Evaluations at follow-up will include blood samples (i.e., CBC with differential, biochemical profile, HIV testing, etc.) and urinalysis and rheumatology consults.