Why is "very allergic" common, but "too allergic" isn't?

[BobStrauss]

Am I wrong, or is there something inherently problematic about using "too allergic"? Is it simply because there is no real acceptable level of being allergic?

[Joe]

1. the words "very" and "too" do not mean the same thing.

Her feet are very small but they are too big for these shoes.
Her feet are very big but they are not too big for these shoes.

2. As you imply, "too allergic" may be illogical as it suggests there is some acceptable level of allergy.

Having said that, it could conceivable be used, logically, in sentences such as:

There is a high pollen count today. You'd better not outside because you are too allergic to the pollen. (ie in some circumstances it might be ok to go outside if you were LESS allergic than you are)