Immune checkpoints are regulators of immune activation, typically expressed on immune cells. They are key proteins that maintain the body's immune tolerance and regulate peripheral immune responses. Immune checkpoints can be divided into stimulatory and inhibitory types. Stimulatory immune checkpoints enhance the immune response against pathogens, while inhibitory immune checkpoints negatively regulate T cell activation. PD-1, PD-L1, CTLA-4, CD28, and OX40 are popular targets for cancer immunotherapy.