CD69, also known as Leu-23, is a type II transmembrane glycoprotein that is expressed on the surface of T cells, B cells, and NK cells. This phosphorylated disulfide-linked 28 to 32-kDa homodimer is constitutively expressed on a subset of thymocytes and platelets. It also acts as an activation antigen of lymphocytes, NK cells, neutrophils, and eosinophils. Studies have shown that stimulation of the T cell receptor (TCR) increases the expression of CD69 on the cell surface. The ability to detect the level of CD69 expression after TCR activation makes CD69 an ideal indicator of T cell activation. The FN50 antibody is widely used as a marker for T cell activation.
