Although the disease context is oncology, this review provides an excellent, detailed overview of STAT3’s fundamental role as a master regulator of the immune system. It highlights STAT3’s dual function: regulating the differentiation of various immune cells (T cells, B cells, MDSCs) while also fostering an immunosuppressive microenvironment in pathological states by restraining effector T cells and promoting regulatory T cells (Tregs). This establishes STAT3’s capability to orchestrate complex immune responses, a key aspect of vascular inflammation. Here, STAT3 is linked to inflammatory response.