When an electron annihilates a positron (anti-electron) the process yields to pure energy in a form of gamma rays, see Electron-positron annihilation. When a proton annihilates an antiproton they produce gamma rays and a swarm of secondary particles, like pairs of top-anti-top quarks. The secondary particles will eventually decay into neutrinos and low-energy gamma rays. Knowing that neutrinos could hold some mass, it could mean that the annihilation doesn't transform all the mass into energy.
For other meanings see: laws of logic, annihilation operator.\n