CD28 (T-cell-specific surface glycoprotein CD28), also known as T44 and Tp44, is a 44 kD disulfide-linked homodimeric type I glycoprotein (PMID: 2162180). It is a member of the immunoglobulin superfamily and is expressed on most T lineage cells, NK cell subsets, and plasma cells (PMID: 2162180, 8386518). CD28 may affect in vivo immune responses by functioning both as a cell adhesion molecule linking B and T lymphocytes and as the surface component of a novel signal transduction pathway (PMID: 2162180, 3021470). CD28 binds both CD80 and CD86 with a highly conserved motif MYPPY in the CDR3-like loop (PMID: 15696168, 7964482). CD28 is considered a major co-stimulatory molecule, inducing T lymphocyte activation and IL-2 synthesis, and preventing cell death (PMID: 1348520).