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CD28 （Cluster of Differentiation 28） is one of the proteins expressed on T cells that provide co-stimulatory signals required for T cell activation and survival. T cell stimulation through CD28 in addition to the T-cell receptor （TCR） can provide a potent signal for the production of various interleukins （IL-6 in particular）.
CD28 is the receptor for CD80 （B7.1） and CD86 （B7.2） proteins. When activated by Toll-like receptor ligands, the CD80 expression is upregulated in antigen-presenting cells （APCs）. The CD86 expression on antigen-presenting cells is constitutive （expression is independent of environmental factors）. It is generally reported, that CD28 is expressed on 50% of CD8+ T cells and more than 80% CD4+ T cells in human, but during the course of activation some T cells lose this molecule. In general, CD28 is a primary costimulatory molecule for T cell activation.