IGLL1 (immunoglobulin lambda-like polypeptide 1) is a critical B-cell development factor with emerging roles in hematologic malignancy and immunodeficiency. As a pre-B cell receptor component, IGLL1 is essential for B-cell lineage development; pathogenic variants cause severe B-cell deficiency ranging from agammaglobulinemia to transient hypogammaglobulinemia 1. In pediatric T-cell acute lymphoblastic leukemia (T-ALL), IGLL1 expression serves as an independent prognostic marker, with high expression associated with significantly improved 5-year overall survival (76.9% vs 47.1%, P=0.018) 2. At the molecular level, IGLL1 acts as a downstream target of the super enhancer-driven gene FYB1, and its knockdown reduces T-ALL cell self-renewal and increases apoptosis both in vitro and in vivo 3. Beyond hematologic disease, IGLL1 has immunodiagnostic applications; recombinant IGLL1 (r-IGLL1) serves as a sensitive (89%) and specific (91%) biomarker for bird fancier's lung serodiagnosis 4. Additionally, IGLL1 expression is significantly downregulated in high-altitude pulmonary hypertension patients, suggesting immune dysfunction in this condition 5. Genetic variants in IGLL1 are also associated with BCG-induced innate immune responses in infants 6.