The Major Histocompatibility Complex (MHC) is considered a system completely defined and only connected with the immune response. However, in addition to the well-known correlation between MHC and the non-self recognition, the MHC region controls a lot of other functions: the recognition of genetic individuality in social relationships, the mate choice and the feto-maternal interplay. Starting from protocordates, the first MHC function was the individual self-identification inside a group, but then it turned into an inter-individual recognition system, which could transmit information about the MHC genotypes. In mammals, the MHC system is functionally and physically linked to the olfactory receptors: when smelling each other, we are able to make a direct genetic analysis through the nose. The MHC individual genetic recognition system plays a fundamental role, both in mate choice and in foeto-maternal selection, from the very start of implantation. All these data suggest that the MHC polymorphism is driven not only by pathogen selection, but also by sexual reproductive-mechanisms. Questions remain about the relative involvement of these two selective forces in MHC evolution.