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T-cell-specific surface glycoprotein CD28, also known as TP44, is a single-pass type I membrane protein containing one Ig-like V-type （immunoglobulin-like） domain. CD28 is one of the molecules expressed on T cells that provide co-stimulatory signals required for T cell activation. It serves as the receptor for CD80 （B7.1） and CD86 （B7.2）. When antigen-presenting cells （APCs） are activated by Toll-like receptor ligands, CD80 expression is upregulated. In contrast, CD86 expression on APCs is constitutive. Notably, CD28 is the only B7 receptor constitutively expressed on naive T cells, highlighting its crucial role in the initial activation of T cells.