Idiopathic CD4 lymphopenia (ICL) is a rare and heterogeneous syndrome defined by a reproducible reduction in the CD4 T-lymphocyte count (less than 300 cells per microliter or less than 20% of total T cells) in the absence of HIV infection or other known causes of immunodeficiency. ICL predisposes to infections and malignancy (summary by Gorska and Alam, 2012).

Immunodeficiency-97 with autoinflammation (IMD97) is an autosomal recessive complex immunologic disorder with variable features. Affected individuals present in the first decade of life with inflammatory interstitial lung disease or colitis due to abnormal tissue infiltration by activated T cells. Patients develop autoimmune cytopenias and may have lymphadenopathy; 1 reported patient had features of hemophagocytic lymphohistiocytosis (HLH; see FHL1, 267700). Some patients may have recurrent infections associated with mild lymphopenia, hypogammaglobulinemia, and NK cell dysfunction. Immunologic workup indicates signs of significant immune dysregulation with elevation of inflammatory serum markers, variable immune cell defects involving neutrophils, NK cells, and myeloid cells, and disrupted levels of T regulatory cells (Tregs). Two unrelated patients have been reported (summary by Takeda et al., 2019 and Thian et al., 2020).