How would an immune system that is allergic to plain water 'know' if you just drunk plain water vs milk or tea?

I read some people have a bad immune reaction if they drink plain water (tap, distilled, sparkling water, etc) and I read about a girl who could not drink even a sip of plain water because it'd send her into anaphylaxis, but if they drink tea, milk or cola they have little to no reaction, so how would their immune cells 'know' if they just drunk plain water? The water molecules floating around in a glass of milk (>90% water) are still water molecules, right? Why would the water/liquid in the body be seen as ''OK'' but plain water is ''not okay'' in these people's immune system cells?