Safety of Belimumab in People With Idiopathic CD4 Lymphopenia and Autoantibodies (Phoebe)

* INCLUSION CRITERIA: Individuals must meet all of the following criteria to be eligible for study participation: * Aged 18-70 years. * Enrolled in study 09-I-0102. * Has a documented diagnosis of ICL, defined as the following: * CD4 count \< 300 cells/microliter in at least 2 separate measurements 6 weeks apart at any point in the past, AND * CD4 count \< 300 cells/microliter within previous 90 days. * Evidence for autoantibody positivity (eg, ANA or in the research flow method looking for antilymphocyte antibodies). * Female participants of childbearing potential must agree to use adequate contraception when engaging in sexual activities that can result in pregnancy, beginning at day 0 (or day 30 for hormonal contraception) until 4 months after the last dose of belimumab. Acceptable methods of contraception include the following: * Hormonal contraception. * Male or female condom. * Diaphragm or cervical cap with a spermicide. * Intrauterine device. * Able to provide informed consent. * Willing to allow samples to be stored for future research. EXCLUSION CRITERIA: Individuals meeting any of the following criteria will be excluded from study participation: * Prior receipt of belimumab. Exception: If an individual had been previously enrolled in this study and received up to 3 doses of belimumab but had to withdraw for reasons that were not related to safety or clinical concerns, they may be re-enrolled. In this case, at least 6 months must have passed from the last prior infusion and they will restart the protocol and drug administration from the beginning. * Allergy to any component of belimumab formulation. * HIV infection or other recognized congenital or acquired immunodeficiency. * Current moderate or severe acute illness (eg, febrile illness, seizure, myocardial infarction, cerebrovascular accident, pulmonary embolism) or progressive serious infection related to ICL that, in the opinion of the principal investigator, would make the participant unsuitable for the study. Pre-existing infections that have been stable both clinically and with laboratory (eg, cryptococcal antigen titer or histoplasma antigen level) and radiographic evaluations on maintenance therapy over at least a year will be eligible. * Untreated hepatitis B or C (acute or chronic). * Active tuberculosis infection. * Serum creatinine \> 1.5 times the upper limit of normal (ULN). * Hemoglobin \< 8 g/dL. * Alanine transaminase or aspartate transaminase \> 2 times ULN. * Serum IgG \< 400 mg/L. * Current use of systemic glucocorticosteroids, with the exception of corticosteroid nasal spray or inhaler and topical steroids. * Any cancer diagnosis or autoimmune condition requiring systemic chemotherapy or immunomodulant-affecting antibody responses (eg, rituximab, ibrutinib), IV or SC Ig supplementation, radiation therapy, or any such treatment within the previous 6 months. Apremilast, Plaquenil, or nonsteroidal anti-inflammatory drugs will not be exclusionary. * Severe depression. Psychiatry may be consulted prior to final eligibility decision. * Infections (recently acquired or exacerbation of a chronic infection) that required new medications for management within the past 60 days. * Receipt of any vaccination within the past 30 days. * Pregnant. * Breastfeeding. * Any behavioral or substance use issue that would compromise appropriate follow up and participation in this study. Concomitant diagnosis of SLE will not be exclusionary. Although SLE can be associated with lymphopenia, patients with lupus and extreme lymphopenia in the absence of cytotoxic or immunosuppressive medications and history of unusual infections will be eligible if they are participants of study 09-I-0102.