PDCD1 and breast cancer: The most well characterized mechanisms outlining BC’s capacity to evade immune destruction are the expression of immune inhibitory co-stimulatory receptors (e.g. programmed cell death protein (PD)-1, cytotoxic T lymphocyte-associated protein (CTLA)-4, lymphocyte activation gene (LAG)-3, the presence of tumor-derived immunosuppressive factors (e.g. TGF-β, IL-10, IDO), and the infiltration of suppressive immune cells (e.g. regulatory T cells (Tregs), myeloid-derived suppressor cells (MDSCs), tumor-associated macrophages (TAMs) in the microenvironment.