MHC-I is extremely polymorphic. In humans, more than 3,400 different human leukocyte antigen class I (HLA-I) molecules have been registered (as of January 2011), and this number is currently growing rapidly as more efficient HLA typing techniques are employed worldwide. The polymorphism of the MHC-I molecule is concentrated in and around the peptide-binding groove, where it determines the peptide-binding specificity. Due to this polymorphism, it is highly unlikely that any two individuals will share the same set of HLA-I molecules thereby presenting the same peptides and generating T cell responses of the same specificities-something, that otherwise would give microorganisms a strong evolutionary chance of escape. Rather, this polymorphism can be seen as diversifying peptide presentation thereby individualizing T cell responses and reducing the risk that escape variants of microorganisms might evolve.