I talked to my sister yesterday and asked her about the pets/allergy thing and earlier when I posted I should have included dogs, I didn't mean to single out cat allergies, it's just that I happen to be allergic to them. Anyway, so I was asking her what would happen if someone who's like REALLY allergic got on the plane and then realized there was a cat under the seat in front of them and they were having really bad reactions and she said if the plane hasn't left yet and the person had a problem they would tell them to talk to the agent and if they had taken off she'd give them an oxygen mask and depending on how bad off they were she'd arrange for paramedics when they land. At that point I started giving her a hard time and threw in rediculous scenarios(cause she's my little sister) and when she started getting pissed off about it I told her I hope I never get on one of her flights and laughed at her.
So ya, not really anything they'll do. lol However, she did say that she's never heard of any problems like that because of pet allergies so I guess they do circulate the air really well.
Oh, then she told me a sad story and the only reason I'm telling you guys is out of warning. One of my sister's flight attendant friends was on a flight and they were getting ready to take off. This lady boarded the plane with her pet cat in it's carrier and went to her seat. I have no idea WHY she would have done this but she let the cat out. The cat took off and made it out the open plane door!

So the lady freaked of course and wanted to get out to find her cat...they told her to go talk to the agent because they won't let people on the tarmac. Reason? Because airplane engines can suck a person up into them and their dead instantly and you only have to be about 15ft away for it to happen. They basically told the lady there was nothing they could do cause either the poor cat had been sucked into an engine or it was long gone by then and no way to find it and as long as it's on the tarmac, there's no looking for it. VERY sad story but let it be a lesson to you, NEVER open your pet's carrier in the plane and if you HAVE to do it, please wait till you are in the air and the door is shut.