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.