Anti-Entrée

I have been spending a lot of time thinking about the meaning of the word 'antipasto'. If it is something eaten before an Italian meal, why isn't it called an entrée, like every other meal? Why do the Italians get a different name for a snack before a meal? What is it anti to? Why pastO and not pastA? Where does appitizer and Hors d'oeuvre sit in all this?