ADA and immunodeficiency disease: There are two ADA enzymes, ADA1 and ADA2, with ADA1 genetic deficiency being the most common cause of severe combined immunodeficiency (SCID) due to buildup of purine metabolites primarily impacting lymphatic cells and ADA2 deficiency precipitating a complex phenotype of inflammation and immunodeficiency (Ehlers and Meyts, 2025; Sauer et al., 2012).