<P> In order for the TCR to bind to the class I MHC molecule, the former must be accompanied by a glycoprotein called CD8, which binds to the constant portion of the class I MHC molecule . Therefore, these T cells are called CD8+ T cells . </P> <P> The affinity between CD8 and the MHC molecule keeps the T cell and the target cell bound closely together during antigen - specific activation . CD8+ T cells are recognized as T cells once they become activated and are generally classified as having a pre-defined cytotoxic role within the immune system . However, CD8+ T cells also have the ability to make some cytokines . </P> <P> The immune system must recognize millions of potential antigens . There are fewer than 30,000 genes in the human body, so it is impossible to have one gene for every antigen . Instead, the DNA in millions of white blood cells in the bone marrow is shuffled to create cells with unique receptors, each of which can bind to a different antigen . Some receptors bind to tissues in the human body itself, so to prevent the body from attacking itself, those self - reactive white blood cells are destroyed during further development in the thymus . </P> <P> TCRs have two parts, usually an alpha and a beta chain . (Some TCRs have a gamma and a delta chain .) Hematopoietic stem cells in the bone marrow migrate into the thymus, where they undergo VDJ recombination of their beta - chain TCR DNA to form a developmental form of the TCR protein, known as pre-TCR . If that rearrangement is successful, the cells then rearrange their alpha - chain TCR DNA to create a functional alpha - beta TCR complex . This highly - variable genetic rearrangement product in the TCR genes helps create millions of different T cells with different TCRs, helping the body's immune system respond to virtually any protein of an invader . The vast majority of T cells express alpha - beta TCRs (αβ T cells), but some T cells in epithelial tissues (like the gut) express gamma - delta TCRs (γδ T cells), which recognize non-protein antigens . </P>

Cd8+ cytotoxic lymphocytes kill target cells as a result of