MHC + X : Springer : 9783642740282 : 3642740286 : 16 Jan 2012 : After the discovery of the function of MHC molecules, namely to provide the context for T cell recognition of foreign antigens, in 1974 Zinkernagel and Doherty made the first drawing of MHC+X (Fig. 1 from Zinkernagel and Doherty, Nature, 251: 547, 1974). Over the next 18 years a very large number of similar drawings ensued, some of real artistic beauty. One side of the problem, the nature of the T cell receptor, was unraveled; however, we still do not know exactly what kind of a structure the T cell receptor recognizes, al­ though in 1987 we learned so much about the structure of MHC molecules and antigen presentation. In schematic presentations no one is now placing the foreign antigen beside the MHC molecule, but rather on top of it, as pointed out by J. L. Strominger at the MHC + X meeting in Paris. The complex of MHC and antigen is named MHC + X, but the precise meaning of this formula remains a "problem perplex", as i