T cells have evolved to be strain specific. For a T cell to respond it has to identify not only the foreign antigen but the antigen must also be attached to a self cell. what is the significance of that? Isn't that the reason behind autoimmunity and hypersensitivity? I understand about foreign antigen, but why self cells? the experiment by Zinkernagel and Dorothy about MHC restriction shows that same antigen presented by a foreign cell will not be killed by sensitized T cells.