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.





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