Allergies, while possibly more realistic (you could very easily imagine it as a more intelligent immune system, that destroys complex entities within the blood, since it doesn't have any effect on chemicals beyond whatever the +2 endurance has), would be a horrible nerf to something that's already got it pretty bad with a) ruining nanites, and b) only protecting against fairly uncommon, though often dangerous status conditions.
Super-aids doesn't really work out because it specifically attacks a normal immune system, whereas Hyperimmune would leave a fairly alien one that would probably fuck with its attack vector.
In other news, I got into my first extended battle with another player today. Some asshole in KING kept hitting on me, and then started insulting me over pages when I told him to fuck off. So I attacked him, and lost by a hair because he had more nanite healers than I did (and better armor, though it wouldn't have mattered if not for the nanites), though my wonderful little rifle otherwise shredded him. He proceeded to die fighting propecia before I could get back to him (probably bled to death, because prop is pitifully weak), so we tried camping his cocoon. Failed, but I came back to the CM to find him fighting propecia again, so I waited and ninja'd the nanite healer from prop, and ran off with it. He later ambushes me, and I kill him, so me and a corpmate camp his cocoon. I actively start hunting him, leaving my ally to guard the cocoon, when he works up the nerve to come back. Then most of KING shows up to save him and his pitiful loot, they burn several nanites keeping him alive, and we haul ass out of there. An entire fucking corp coming to ward off two low level players. XD
The best part was the retarded trash talk he kept spamming to us the whole time.