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CD28, a 44-kDa disulfide-linked glycoprotein, is constitutively expressed on the surface of most CD4+ and CD8+ T cells. It plays a crucial role in T cell activation and signaling by interacting with its ligands CD80 （B7.1） and CD86 （B7.2） on antigen-presenting cells. CD28 enhances T cell receptor signaling, promotes cell cycle entry, and supports the survival of T cells by upregulating anti-apoptotic proteins like Bcl-2. It also preferentially promotes TH2 differentiation, aiding B cell germinal center formation and isotype switching. CD28 signaling involves multiple pathways, including PI3K, Grb2, and NFκB activation, which collectively regulate T cell proliferation, differentiation, and effector functions.