CD8 T cell surface antigen is heterodimer of an alpha and a beta chain linked by two disulfide bonds .It belongs type I membrane protein. Selectively expressing of CD8 on a subset of T cells leads to CD8 T cell development. Through identifying cytotoxic/suppressor T-cells that interact with MHC class I bearing targets, CD8 is thought to play a role in the process of T-cell mediated killing. Veillette et al (1988) found the CD8 is associated with the internal membrane tyrosine-protein kinase p56lck.