CD28, also known as the T-cell-specific surface glycoprotein CD28, is a protein encoded by the CD28 gene in humans. It is a glycoprotein that is constitutively expressed on both resting and activated T cells. CD28 plays a crucial role in T cell activation and signaling, providing essential co-stimulatory signals required for T cell activation, proliferation, and cytokine production. It enhances the production of IL4 and IL10 in T cells in conjunction with TCR/CD3 ligation and CD40L co-stimulation. CD28 interacts with its ligands, CD80 （B7-1） and CD86 （B7-2）, which are expressed on antigen-presenting cells, to mediate the co-stimulatory signals necessary for T cell activation. This interaction is critical for the early events of T cell activation and the decision between immunity and anergy.