This thread may get heated as the allergy topic typically does. From the legal side I agree that blindness and many other conditions are legally protected disabilities and airlines are required by law to accommodate the passenger. Allergies and other medical conditions are not.
Someone with bad allergies can (and should) take precautions to minimize their chance of problems. Medication, epi-pen, mask, respirator, etc. Really, how do you know you are not going to run into a dog/cat in a train, bus, store, or just walking down the street. A blind person cannot do anything to mitigate their blindness.
My belief is that if your condition is so bad that it could cause death or serious injury, and you are unable or unwilling to find a way to minimize the risk, you should not be traveling by commercial transport. I don't see it as any different than someone who is at high risk of DVT expecting the airline to give them a business class seat.
I'm not saying the airline should do nothing, but expecting them to guarantee a flight is allergen free or remove six seats so your hospital gurney can be accommodated is too much.
Going OT a bit, is it possible for a human to be allergic to other humans? We share pretty much the same hair/dander/skin oils as our mammal cousins so it seems like it could be possible. How would an airline handle that one?