TBDs are a group of inherited conditions that are caused by germ line mutations and that effect telomere maintenance. These disorders are associated with progressive multiorgan dysfunction, including bone marrow failure, pulmonary fibrosis, and immunodeficiency. A defining feature of the immunodeficiency in adults with TBDs is lymphopenia, particularly involving low CD3⁺ and CD4⁺ T-cell counts. As shown in the left panel (blue box), lymphopenia was associated with increased infections and significantly worse overall survival. Furthermore, low lymphocyte counts correlated with higher rates of bone marrow failure and solid tumors, as illustrated in the right panel (pink box). These findings highlight the need for further investigation into how lymphopenia contributes to marrow failure and impaired tumor surveillance in TBDs.