PD-1, also known as CD279 (cluster of differentiation 279), is a cell surface receptor that belongs to the CD28 immunoglobulin super family and is expressed on T cells and pro-B cells. Studies have shown that PD-1/PD-L interaction function as an immune checkpoint for induction and maintenance of T-cells involved in peripheral tolerance and protects tissues from autoimmune attack. PD-1 performs dual mechanism of promoting apoptosis (programmed cell death) in self antigen specific T-cells in lymph nodes, while simultaneously inhibiting apoptosis in regulatory T cells (suppressor T cells).