Thymic function as a central determinant of immune competence and clinical outcomes.

The thymus is a central regulator of immune competence, sustaining the continuous generation of a diverse and self-tolerant T-cell repertoire throughout life. Although traditionally regarded as primarily relevant during early development, accumulating evidence highlights its persistent role in shaping adaptive immunity across physiological and pathological conditions. In this review, we examine the role of thymic function as a determinant of immune competence and treatment efficacy across conditions in which adaptive immunity is challenged or therapeutically reshaped, including hematopoietic cell transplantation, severe infections, autoimmune diseases, immune checkpoint inhibitor therapy, and aging. In these contexts, impaired thymic activity is consistently associated with contraction of the naïve T‑cell pool, reduced T‑cell receptor (TCR) diversity, increased susceptibility to immune‑related complications, and inferior clinical outcomes. We then provide a critical overview of the methodologies currently available to assess thymic function, including flow-cytometric identification of recent thymic emigrants, molecular quantification of T‑cell receptor excision circles, analysis of TCR repertoire diversity, and imaging‑based approaches, highlighting their respective strengths, limitations, and translational applicability. Finally, we discuss the emerging relevance of thymic function in the context of adaptive and T‑cell-redirecting therapies, where sustained immune competence depends on the integrity of endogenous T‑cell regeneration. Collectively, this review positions thymic function as a clinically actionable biomarker for immune monitoring, patient stratification, and personalization of adaptive immunotherapies.