Imp is bugged

I've played on Warmane before and have just recently returned on Icecrown . I've just rolled my first Warlock here and as soon as i got my imp the problem manifested itself . The imp doesn't take any part in combat by itself ( you have to manually activate its fireball spell ) and the most worrying thing is that when the imp is attacked it doesn't take any damage ( my Warlocks health bar reduces instead ) . I've put in a bug report about this issue but I wondered if anyone else has any insight about this issue ?

If you want pets to "take part in combat by itself" you need to put it on aggressive which is a bad practice. Make a petattack macro.
And i can't reproduce my warlock taking damage when imp is hit so I'm unsure of how you've got that to happen.

No you misunderstand , when you click on attack your imp should automatically start hurling fireballs at the target or if you attack it should join in but it does neither , and as for me being damaged when the imp is attacked I can assure you it is nothing that i have done . I'm currently playing Warlock in retail and classic and have never experienced this problem before .

Then you need to put it's fireball on Autocast so that it casts the spell each time it's off CD (in case of fireball after each cast, it's got no CD). Autocast is disabled by default for all abilities.
Right click it to make it autocast and it will cast it while it's in combat.