A rare primary immunodeficiency disorder characterized by persistent CD4 T-cell lymphopenia (less than 300 cells/B5L on multiple occasions) not associated with any other underlying primary or secondary immune deficiency. Patients typically present opportunistic infections (with cryptococcal, mycobacterial, candidal, varicella zoster virus infections and progressive multifocal leukoencephalopathy being the most prevalent), malignancies (mainly lymphoproliferative disorders), or autoimmune disorders. Some individuals are asymptomatic and incidentally diagnosed. The disease is idiopathic CD4 lymphocytopenia (Monarch Disease Ontology id MONDO_0014226). Also known as: immunodeficiency type 13.