CD28, a well-known costimulatory receptor expressed on T cells, is a type I transmembrane glycoprotein of the Ig superfamily. When the T cells receive the first signal delivered by APCs (antigen-presenting cells), it initiates signaling cascades, which favor T cell survival and proliferation. It has been demonstrated that patients with lung cancer who responded to PD-1 therapy had more CD28+ T cells, suggesting that CD28 may predict treatment response.