Why is the H2O molecule an antigen despite its tiny size and simplicity?

I just saw a news article about a woman with Aquagenic Urticaria which basically means she produces antibodies against H2O molecules which causes very bad symptoms if she touches or drinks water.

 

https://www.thefreelibrary.com/JUST+ONE+CUP+OF+WATER+COULD+KILL+LITTLE+HEIDI%3B+Girl's+deadly+allergy…-a061152595

https://socialnewsdaily.com/75296/woman-possesses-fatal-water-allergy/

 

We’re told you can’t be allergic to oxygen, glucose, salt etc because they’re too small and simple. So why is H2O an antigen?

NCBI source - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3276800/

Double posting is a no-no