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I'm writing a drama book and it's meant to be taken 100% seriously, and the main character was born with an allergy to $H_2O$. She does not have Aquagenic Urticaria, as that only affects the skin. She has a peanut allergy-esque reaction to any water she drinks, or gets in her bloodstream or organs. This happens regardless of if the water is medical grade distilled water. For example, in the story she accidentally takes a sip of water and goes into anaphylactic shock, but is saved by her parents, who carry epi-pens for her.

When she was 8, she had to get her appendix removed, and doctors had to re innovate surgery just for her so that $H_2O$ never came into contact with her bloodstream or internal organs, but later on, a nurse gives her intravenous saline and then she almost dies of anaphylactic shock since the saline was water based, and because $H_2O$ was now present in her bloodstream, causing her immune cells to release histamine.

Doctors affirm that she produces antibodies against the $H_2O$ molecule, so her immune system assumes that $H_2O$ is foreign and starts attacking her body. She has to drink only whole milk and orange juice to survive. As long as she does not drink or come into contact with water, she is perfectly healthy and has no symptoms.

Is this realistic?

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    $\begingroup$ The human body is over 50 percent water. So you should hope that someone will be able to explain why your character isn't allergic to herself. $\endgroup$ – M. A. Golding Jun 29 at 16:14
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    $\begingroup$ @Warden Sodium chloride is not just a mixture, it's a chemical compound. You have Na⁺ and Cl⁻ in there. It's Na⁰ and Cl⁰ that are dangerous, as they "really want" to be Na⁺ and Cl⁻. When they mix, they react, oxidizing sodium and reducing chlorine, forming the deadly neutral atoms into perfectly safe ions. $\endgroup$ – Hearth Jun 29 at 16:20
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    $\begingroup$ Most of mammalian bodies are indeed mostly plain water contained by cell walls, not water bound in complex organic molecules. There's also a big problem of the mechanism to trigger allergic response, since water won't trigger antibody production. I have seen no credible evidence otherwise. If you don't drink water, you die in a few days - it's that simple. $\endgroup$ – user535733 Jun 29 at 16:35
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    $\begingroup$ Simply look up Wikipedia on how immune responses work. Lots of substances cannot trigger immune responses - the molecules are simply wrong shape (and water is among them). Diet Coke is entirely plain water with a bit of fizz, coloring, and flavoring. Folks who claim "water allergies" might be complaining about a polluted water supply or other problems, or they might be cranks or con artists. $\endgroup$ – user535733 Jun 29 at 16:41
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    $\begingroup$ Unverified claims in the news are cheap. If common, there would be dead bodies to examine. Smells like a post-Flint scam of some kind to me. "A lot of people getting into the news" seems to be something local to you - it's not happening here. $\endgroup$ – user535733 Jun 29 at 17:09
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It makes sense as absurdist allegory.

If meant as scientific truth it will not work. There is too much water in us and on us. What would she drink? Everything we drink is mostly water. Our blood is mostly water.

But you could make it work, especially as the central tenet of the story. Assert it up front. Build the story around this unusual and difficult truth. This could be absurdist allergory. I think of Kafka's The Metamorphosis.

https://www.kafka-online.info/the-metamorphosis.html

One morning, as Gregor Samsa was waking up from anxious dreams, he discovered that in bed he had been changed into a monstrous verminous bug. He lay on his armour-hard back and saw, as he lifted his head up a little, his brown, arched abdomen divided up into rigid bow-like sections. From this height the blanket, just about ready to slide off completely, could hardly stay in place. His numerous legs, pitifully thin in comparison to the rest of his circumference, flickered helplessly before his eyes.

‘...

‘O God,’ he thought, ‘what a demanding job I’ve chosen! Day in, day out on the road. The stresses of trade are much greater than the work going on at head office, and, in addition to that, I have to deal with the problems of traveling, the worries about train connections, irregular bad food, temporary and constantly changing human relationships which never come from the heart. To hell with it all!’

People do not turn into giant insects. A person who did would probably not spend time in bed thinking about his demanding job. In the story, everything else is normal for our world. Only the transformation is magical and it is established in the very first sentence. We realize as we read that the transformation is to be understood as representative of something else; a metaphor about the protagonist and his own self worth and sense of place.

Water allergy would work for that. An allergy to water is so outré that no-one will think maybe that could actually happen to someone. In Metamorphosis, Gregor might have awoken with a growth or a tumor and that would be plausible, but to awaken as a beetle immediately tells you what sort of situation is going on in this story. Your characters water allergy is a metaphor for her tenuous and difficult place in her family and in the world

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By straight up biology, this won't work. The issue is that the way antigens work does not permit the response to H2O you seek without instantly killing the host as it turns on itself. Antigens work using known chemistry. The specificity you seek is just not feasible. An analogy for how difficult it might be would be to set a bonfire with a mixture of wood types, and grab only ash particles generated by Oak using only your bare hands. The subtle bonding between water molecules just isn't expressive enough to be detected by antibodies before the liquid gets mixed with your existing liquids (such as stomach acid or sweat), adulterating it on par with orange juice or milk.

However, if we broaden the diagnosis a bit, we might be able to get something. Let's not have a straight up allergic reaction to H2O. Instead, le'ts have another reaction to H2O. Say your main character responds to water that is too pure. Pure water destabilizes some of her cells in some way, but the presence of a sufficient volume of other compounds suppresses this effect. The effect of this destabilization is that the cells begin quickly producing compounds to mix with the water to get it as far from pure as possible. Then have her be allergic to whatever these compounds are. Or, for more realism, have the cells deplete their stores of this compound, and then release a signalling hormone which she has an autoimmune response to.

This would lead to distilled water being far more dangerous for her. Koolaid or Lemonade may be less dangerous, and as you approach liquids like milk and orange juice, you have enough stuff in the water to keep her reactions at bay.

This would then avoid all the messy trouble with the fact that we have lots of water all over our body. That water is already blended into blood, lymph, cytoplasm, urine, etc. It would be impure enough to remain in balance. However, a quick drink of pure water or a splash of water on the skin might require enough work to adulterate that the chemicals produced generate the allergic reaction.

This also has a convenient kernel of truth in the middle. Pure water, in large volumes, is toxic. If your body uptakes too much water (it's hard to do on purpose), the sodium concentrations in your blood drop accordingly. This causes cells to release sodium, and the process of doing so can lead to things like arrhythmia and brain swelling that can kill. However, if you were to drink large volumes of water with sufficient electrolytes in them, you would never suffer this effect because the body would remain in balance.

Brawndo

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    $\begingroup$ Dang nabit! I was writing a hugely similar answer only to realize it was what you had already written. I hate being late to the party. +1! $\endgroup$ – JBH Jun 29 at 18:37
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Blood is a water based solution.

In vertebrates, blood is composed of blood cells suspended in blood plasma. Plasma, which constitutes 55% of blood fluid, is mostly water (92% by volume).

Cells are full of water.

Milk is also a water based solution. Same holds for orange juice.

She has a peanut allergy-esque reaction to any water she drinks, or gets in her bloodstream or organs.

If this was true, even admitting that for some weird reason your character managed to leave the womb of their mother, they would have been killed by the first sip they'd took of motherly milk.

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    $\begingroup$ Just something I'm curious about. Would the H2O being combined with other chemicals (ie milk proteins, blood proteins, etc) nullify such a reaction? For instance, sodium and chlorine are deadly and toxic by themselves, but when mixed together they form harmless sodium chloride which we eat daily as table salt. I'm pretty sure the body is not pure water by itself. $\endgroup$ – Warden Jun 29 at 16:19
  • $\begingroup$ @Warden Salt is not totally "harmless". I used to love salt but have hypertension and so have cut down on salt use for years since sodium raises blood pressure. So someone could claim that sodium and chlorine are fast acting poisons and salt is a slow acting poison. $\endgroup$ – M. A. Golding Jun 29 at 16:28
  • $\begingroup$ What I was saying is when two chemicals are mixed they have vastly different properties, and in the story my character says that milk doesn't kill her because of its chemical composition. $\endgroup$ – Warden Jun 29 at 16:32
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    $\begingroup$ If you mix sodium and chlorine together, you get a pile of explosive powder which, if you breathe over it, kills you. Combining them is not enough. It's not until the sodium and chlorine bond together with chemical bonds that they become sodium chloride (table salt) which is not deadly. While we do bond H2O to form chemical compounds, only a small portion gets bonded that way. The majority is left in aqueous form because we use that water as a solvent to transport other chemicals we need. $\endgroup$ – Cort Ammon Jun 29 at 16:58
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No, it's not possible.

A true allergy is mediated by histamine (released by the mast cells) and involves an "immune system-mediated adverse reaction to food proteins." (Or to non-food proteins in the case of other allergies.) While many sources assume IgE is the only type of true allergy, it can also be IgG, IgA, and others.

Water doesn't have proteins, unless they come from contamination (or from being in a natural state such as a creek or water faucet or in blood).

A pseudo-allergy is a very real condition where something causes release of histamine (or histamine can come from external sources) and the end result is very much like an allergy. Aquagenic Urticaria could be classified this way.

Then we have intolerance (food intolerance or others). Allergies and pseudo-allergies are types of intolerance. This simply means (the inclusion of) some medical reaction that isn't mediated by histamine. Lactose intolerance and celiac disease are examples.

Your character might be intolerant of water (something that a layperson could easily call an allergy) but, as others have pointed out, that doesn't work. Water is most of our bodies and ubiquitous.

I've known people in real life who were "intolerant of water" but it's always about contaminants in water. In the case of people with extreme reactions, it's generally about plastic. But you've ruled this out as a possibility.

So what you're left with is a situation that isn't real but can still be part of a fantasy story. This idea that somehow if a cow drinks water and produces milk, that renders the water safe for your character. Or if an orange tree produces fruit after rainfall, somehow the tree is able to convert the water into a safe juice (I'll assume that your character doesn't drink the majority of orange juice brands which are orange juice concentrate reconstituted with water).

The novel Wicked does this. It's a take on the Wizard of Oz story, told from the point of view of Elphaba, the Wicked Witch of the West. As anyone who has read the books or seen the movie knows, Dorothy kills the witch by pouring water on her, causing her to dissolve. In this novel, Elphaba has a lifelong reaction to water. She bathes herself by rubbing her skin with oil and scraping/wiping it off (also done in some ancient cultures).

[Book version] Elphaba is also unusual in that she is apparently allergic to water, and avoids touching it at all times, never crying or bathing. She cleans herself by rubbing oil into her skin. She has a power that she cannot control. This shows mainly when she is angered. An example is when she sees Chistery trapped on an island in the middle of a lake. Ignoring her allergy to water, she jumps into the lake to save the monkey. However, when Elphaba touches the water it turns to ice for her...the weather changed to suit Elphaba's needs. (ref)

So if you're looking for a scientific explanation for an "allergy" to pure unadulterated water in someone who not only lives but can drink milk and orange juice, you will be disappointed. But if you want to write fantasy where a character can not contact or consume pure water, or substances to which pure water has been added, you can do as you like.

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    $\begingroup$ +1 for best science based, reality check answer! Sadly, if the proposed scenario is not good science fiction, it is likewise horrible fantasy. $\endgroup$ – elemtilas Jun 30 at 1:52
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    $\begingroup$ @elemtilas Thanks. Though I will say that the premise works in Wicked. It's just enough of a mix of realism and fantasy to hold. If you don't think about it too hard. $\endgroup$ – Cyn says make Monica whole Jun 30 at 3:14
  • $\begingroup$ The point being, I suppose, you'd have to shut your brain off... Never read Wicked, but in Baum, from what I've gleaned, it's reasonably clear that at least the Wicked Witch of the West is in some way "dessicated" by her evil nature. When Dorothy dumps a bucket of water on her, it's not that the Witch is allergic to it, but that its purifying nature washed her evil nature away. $\endgroup$ – elemtilas Jun 30 at 17:57
  • $\begingroup$ @elemtilas Wicked is worth reading. It's a completely different take. The "wicked witch" is not in fact evil in this version. My description of the "allergy" is accurate, but only for the book Wicked (I haven't seen the musical but it treats it differently). $\endgroup$ – Cyn says make Monica whole Jun 30 at 18:00
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IMHO that is not realistic because the human body is over 50 percent water and blood plasma is similar to water. Thus she should have been allergic to herself and to the amnoitic fluid in her mother's uterus and been stillborn.

It is true that many allergies develop or grow stronger some time after a person is born. So she could develop her allergy as a child and discover it by accident when something she's done a thousand times before is suddenly painful. Since she might not react instantly to water and water allergy would probably not be a first guess discovering what substance she is allergic to could take a long time and she might not survive that long.

But it is possible that someone with more knowledge of biology will be able to think of a very clever way to have your character be allergic to water and still be able to survive. And if anyone can do so you should be very grateful to them.

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