DPP-4 enzyme has been implicated to have a direct effect in T lymphocyte regulation [18], and DPP-4i suppress mitogen-stimulated T-cell responses [31], which leads to an increased incidence of infections (e.g., upper respiratory tract infection, nasopharyngitis, and urinary tract infection) [20, 32]. Here, DPP4 is linked to Recurrent upper respiratory tract infections.