Just a question. A particle and a corresponding antiparticle (e.g. electron and positron) can annihilate by mutual interaction, producing energy (photons). If the process of mutual annihilation occurs necessarily between particles and antiparticles (matter and antimatter), we would expect that - for example - an electron and an antiproton could suffer this process. However, they don't. We know that the annihilation process occurs between particles/antiparticles of the same family - in some way, we could say 'between a dual pair' (particle and corresponding antiparticle). However, this restriction seems to me very weak; I don't glimpse a convincing law, neither a "strong physical support" for this. The mere division (classification) of the elementary particles by their strongly abstract properties (spin, isospin, strangeness, etc.) seems to be more properly a rule than a very "touchable" reality. Then, the above restriction seems to be incomplete: I feel a lack of supporting information. This is the question: what is the "strong reason" for particle-antiparticle annihilations occur only between dual?