1. Documented history of idiopathic CD4 lymphocytopenia as defined by CD4 T cell count <300 cells/microL or <20% of total T lymphocytes on 2 occasions at least 6 weeks apart in the absence of any illness or medications accounting for CD4 lymphocytopenia. Although the protocol will primarily enroll ICL patients who are lymphopenic at the time of enrollment, up to three patients who had clear documentation of ICL in the past and are currently not lymphopenic may still be enrolled for comparative purposes.