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In my world, vampires need to consume a small quantity of human blood every month to survive. They usually supplement their diet with animal blood and human food.

A vampire which goes without human blood for a few months dies.

Vampires are not magical beings. They are the result of genetic mutations and have evolved alongside humans.

Vampires have a beating heart and a working circulatory system. They are also faster, stronger and have better senses than the average human.

I am looking for a reason why human blood (as opposed to blood of other animals) is essential for vampires.

I don't think my question is a duplicate of Why can vampires only drink human blood? because vampires in my world do not subsist solely on human blood.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Monty Wild ♦
    Commented May 17, 2021 at 14:54
  • $\begingroup$ Why can't silkworms eat lettuce? There's a protein that can't be synthesized by the vampires, present in human blood in sufficient quantities, but the versions in other primates differ enough as to be incompatible. This protein is absorbed by a new (small) organ in the upper stomach and esophagus, and thus isn't digested by the stomach in a way that would destroy it. $\endgroup$
    – John O
    Commented May 17, 2021 at 15:53

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Immunology:

The need for human blood is not, really, about food. It's about antibodies.

The vampire's immune systems have come to function in a completely different way from human immune systems. This has allowed them to enter into a symbiotic relationship with several bacteria that block cancers, promote muscle oxidation at amazing levels, allow stem cells to rebuild the body repeatedly, etc.

Only the vampire is unable to regulate the bacterial growth. The same immune system that allows them to tolerate the bacteria renders them incapable of resisting them. With enough time, the bacteria will flood the vampire's body and eat them alive.

The bacteria, however, are harmless to normal humans, who are universally immune to it. The various symbiotic bacteria are related to a common gut bacteria and everyone has antibodies to it. These are the same gut bacteria that cause people to develop antibodies to A & B antigens on human red blood cells. Animal blood has antibodies, but the vampire's immune system recognizes animal antibodies as foreign and rejects them, while tolerating human antibodies.

Whenever the vampire senses the bacteria are growing out of control, they are compelled to seek out human blood. While the vampire can't make the antibodies, they CAN use them. The more human blood a vampire consumes, the more the bacteria are in check, and the more "human" the vampire is (tolerating sun, ordinary strength, possibly even aging). The less human blood the vampire uses, the stronger and more vampiric they get (right up to the point where they die of sepsis).

This does mean the vampires would have a unique vulnerability or gift - antibiotics. This could be anything from a replacement for blood (the wonders of modern science) to an acute vulnerability (drink the wrong victim's blood and your bacteria die, and then YOU die). This can be whatever you want to make of it story-wise. Eventually, the bacteria will develop resistant strains and the antibiotics will stop working.

This also means that blood type could affect the vampire. Perhaps O blood is the universal treatment, but B blood (alone) leaves powers more intact (yet only as a temporary fix), and A blood (alone) weakens the vampire while not entirely satisfying their needs.

It would also mean the vampires could get everything they need from blood by using/consuming/transfusing plasma. In the same way modern medicine uses convalescent plasma to treat infections, your vampires just need the antibodies. Eventually, this will allow vampires to take synthetic antibodies and skip human blood altogether.

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    $\begingroup$ Isn't that going to mean the vamps are going to have to cross type and match each time they go for a drink? Sucks to be the vamp with AB blood. $\endgroup$
    – Aron
    Commented May 18, 2021 at 1:39
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    $\begingroup$ @Aron They would only need to do an ABO typing if 1.) the typing actually matters and 2.) only if they are looking for a specific effect. But it might mean they have stock "go-to" people who's blood is predictable in it's effects, while others are less satisfying for their needs. Odds are, they'd bite people and go back to the ones who were most useful. $\endgroup$
    – DWKraus
    Commented Aug 24, 2021 at 21:47
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T Cells

A less fantastic version of DWKraus’s excellent answer. Your vampires have evolved a symbiotic relationship with humankind. Vampires can produce their own antibody-crafting B-cells, and can produce their own antigen-library T-cells, but vampire T-cells have evolved to lose the ability to recognize new antigens.

Vampires drinking a small amount of human blood every few months take advantage of a symbiotic tweak vampires have with their evolutionary cousins. Human T-cells escape digestion, passing directly into the blood where vampire T-cells prefer the human version, and use it as a template.

Diseases are changing constantly as slight mutations are sufficient to escape detection by the old definitions. This is why we have four or more flu vaccines every year, just for the top-4 most prevalent mutations for the year. As evolved without the ability to recognize the new mutations, vampire defenses against infection quickly are dated to the point of irrelevance.

This answer maybe improves on DWKraus’s by requiring only a little amount of human blood (just enough to get some updated antivirus definitions from their symbiote).

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    $\begingroup$ Actually, babies obtain antibodies consuming their mother's milk. It's completely within the realm of possibility that these vampires have adapted this mechanism into adulthood. Thus the vampires regulate their immune system through diet. Vampires=human relatives/offshoot. For your answer, the vampire cells would be creating something like a hybridoma cell. en.wikipedia.org/wiki/Hybridoma_technology $\endgroup$
    – DWKraus
    Commented May 15, 2021 at 17:01
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    $\begingroup$ Nitpick: How do humans profit when vampires suck their blood? If order for relationship to be symbiotic, both species need to benefit - otherwise (when only one of the species benefits) it is parasitic (and not symbiotic) relationship instead... $\endgroup$
    – Matija Nalis
    Commented May 17, 2021 at 14:36
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    $\begingroup$ @MatijaNalis Maybe the vampires are basically steering/farming the humans? They intervene before full-blown crises, avert disasters, drop hints for technical improvements. Like ants taking care of antferns, getting rid of any insects or fungi damaging the host. Or like cows (out of aurochs) and sheep/goat have multiplied enormously by human practices $\endgroup$
    – user3445853
    Commented May 17, 2021 at 16:21
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    $\begingroup$ @MatijaNalis there are more than one type of symbiotic relationship. You're thinking of the mutualistic type, which benefits both. There are also commensalistic (where one benefits and the other is unaffected) and parasitic. $\endgroup$
    – Praearcturus
    Commented May 18, 2021 at 0:31
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They're all junkies.

They're not after the blood, they're after the endorphins in the blood. Their own system uses a slightly different class of hormone to fill the role endorphins do in ours, but they've retained the opioid receptors they inherited from their ancestors which gets triggered by the endorphins in their prey's blood, which give them one hell of a high. The downside is while their opiod receptors give them a hit far above what a human would experience, it results in severe physical addiction and potentially fatal withdrawal. They can obtain some from non-human regular meat, but all that does is delay the onset of the withdrawal.

None of this was known, of course, in their history, so they assumed they needed the blood to live and thus made sure their offspring received it, which they believed was necessary to survive. This unknowingly addicted their children, and the cycle continued. All it would take to break the cycle is not drink blood from the start, but there's two things acting against this: the first is that not many know the truth and so there's still this folk belief they need to consume blood. The second is that some of them do know the truth, but they don't want to give up the high and so they actively suppress it. If people knew there was an alternative, then all of a sudden what they still want to do isn't a matter of survival but of personal choice. It's easier for people to accept your little kinks if it's something they believe everyone has to do.

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Best I can think of is DNA.

You said it yourself that your vampires feed mostly on the blood of other animals much like a vampire bat, but vampire bats are in simply for the nutrients in the blood (which aren't all that many, requiring the animal to feed regularly and one of the reasons they'll die if they go longer than 3 days without a proper blood meal), so out of the "bat" we can see it must be related to something other than nutrients.

Therefore your vampires need something else specifically from humans and which no other species has. My best guess for why they'd need it? Your vampires need the human genome itself.

According to this forensic site from the University of Arizona:

Blood is an excellent source of human DNA. DNA is present in white blood cells of humans, but not red blood cells which lack nuclei. A dime-sized spot of blood, approximately 50 µl in volume, is enough DNA for a typical VNTR analysis.

This fact, coupled with the fact that vampires are a result of genetic mutations that evolved alongside humans lead me to the following conclusion: for whatever reason, at the cost of enhanced senses and abilities when compared to normal humans, vampires have an unstable genome, and rely on human DNA to maintain their bodily functions, so, similarly to how rotifers incorporate the genes of plants, fungi and bacteria they eat to ensure their genetic diversity and remain capable of thriving and ensuring genetic diversity despite reproducing via cloning, your vampires incorporate the human DNA found from the white blood cells ingested with the human blood, using it as a way to repair certain parts of their own DNA, which are almost identical to the same parts found in normal human DNA but also seem to "degrade" over time for unknown reasons (be it mutation, deletion or any other process), these parts cease to code certain enzymes vital for the vampire's survival, and need to be replaced by a healthy counterpart copied from normal human DNA every once in a while, in a process similar to what's seen with CRISPR/CAS-9. If a vampire fails to consume human DNA, these structures will degrade to the point it can no longer relay the codes its meant, resulting in the lack of production of certain proteins and enzymes vital for their survival, culminating in their bodies shutting down for good.

As to how they would perform this separation of components before sending what's useless for digestion, I'd suggest that the vampires have a special stomach divided in 2 chambers, a smaller chamber in which the white blood cells are separated for posterior DNA "extraction" by the vampire's natural "genetic editing" system and a main chamber where what's not needed by the vampire in its current state is digested as it would be in a "normal" stomach.

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  • $\begingroup$ Wouldn't the human DNA be instantly degraded / denatured by the acid in the stomach? $\endgroup$
    – AlexP
    Commented May 15, 2021 at 14:06
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    $\begingroup$ @AlexP they'd probably have processes to prevent that from happening and separating the white blood cells before initiating digestion, which funny enough is an internal mechanism whose existence is necessary for both answers to work, except DWKraus thought about using the white blood cells themselves while I thought to use only the human DNA within $\endgroup$
    – ProjectApex
    Commented May 15, 2021 at 14:15
  • $\begingroup$ The antibodies in @DWKraus's answer are dissolved in the plasma. His answer has nothing to do with the white blood cells. Plus, it is known that (freshly born) humans and do take antibodies by mouth. Moreover, the question explicitly says that the vamp pyres eat other stuff too, and therefore they must have a pretty normal digestive system. $\endgroup$
    – AlexP
    Commented May 15, 2021 at 14:33
  • $\begingroup$ @AlexP as would be the white blood cells. Either way the vampire needs a method to prevent the components they need in the human blood to be separated so that they're not digested, such as a pre-stomach chamber. Also the answer does say they supplement their diet, but is written in a way that emphasizes that the consumption of blood is still important to their diet, which leads me to believe human food is secondary when compared to blood, and that their digestive track and kidneys do have adaptations for a diet a tad more centered about digesting blood, like those found in vampire bats. $\endgroup$
    – ProjectApex
    Commented May 15, 2021 at 14:55
  • $\begingroup$ I'll answer to that comment under your answer. " human blood would need to have, at least, a fairly large number of exclusive traits to it," - that is indeed a good and correct statement, and human blood indeed has plenty of own traits, because if u think about if, if it would not to then we would have 0 problem/difficulties with donor blood. And there is more, not only unique traits can be important, but typical proportions of more common traits can be important, but it even harder to explain why so. I just answer u q, and not so much interested in topic, u get it or not, but i said enough. $\endgroup$
    – MolbOrg
    Commented May 15, 2021 at 21:06
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Those guys have aquagenic urticaria (think water allergy). They can ingest regular water, but it will harm them. Over time the harm builds up.

Mammalian blood, for some reason, does not trigger the allergic response, so it is the only safe thing for them to drink if they want to stay hydrated. Properly hydrated vampires will drink blood every day, not just once a month. They will also be healthier and stronger.

If you change the part where they consume animal blood to consuming only primate blood, you can also blame a tick-induced red meat allergy. There is this tick in America which can induce this allergy with just one bite - if you catch it, you will have a reaction to any meat with alpha-gal in it. Among mammals, it seems that only primates don't have alpha-gal in their organism (which is why you even get this condition from the bite). Then it becomes a matter of your vampires needing meat in their diet, and unless they would consume the chimps in a zoo the only safe meat for them is people. They think they need the blood but what they actually need is meat. Turns out they like their steak raw and bleeding.

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    $\begingroup$ That alergy, whaaat, man nature is hard maan, eh, lol $\endgroup$
    – MolbOrg
    Commented May 15, 2021 at 18:58
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Scurvy and "Vitamin H" and blood pressure reduction from Sunlight

Whether it's the immune cells, particular strands of DNA or the exact composition... Human blood contains something lacking in other species.

Examples exist elsewhere of spieces that contain something others don't... Lobsters famously contain stuff that are useful all around the place.

https://www.nhm.ac.uk/discover/horseshoe-crab-blood-miracle-vaccine-ingredient.html

What is horseshoe crab blood used for? Horseshoe crab blood is bright blue. It contains important immune cells that are exceptionally sensitive to toxic bacteria. When those cells meet invading bacteria, they clot around it and protect the rest of the horseshoe crab's body from toxins.

https://www.statnews.com/2020/02/19/lobster-blood-could-play-role-in-new-drugs/

Scientists with the company have found that hemocyanin, a protein in the fluid, works as a powerful stimulant for the immune systems, Bayer said. For example, experiments show the substance can reduce the viral load of herpes simplex virus-infected cells, according to documents the company filed with the U.S. patent office.

What does human blood do? As importantly... what's in human blood not in vampires? Sun light.

https://www.thoughtco.com/facts-about-blood-373355

11 UV Rays Reduce Blood Pressure
Exposing a person's skin to the sun's rays reduces blood pressure by causing levels of nitric oxide to rise in the blood. Nitric oxide helps to regulate blood pressure by reducing blood vessel tone. This reduction in blood pressure could cut the risks of developing heart disease or stroke. While prolonged exposure to the sun could potentially cause skin cancer, scientists believe that very limited exposure to the sun could increase the risks of developing cardiovascular disease and related conditions.

Simply put... without a dose of Vitamin H to limit blood pressure, Vampires suffer like those without Vitamin C suffer due to a lack of something important. In this case, Nitric oxide in specific levels and forms, is missing from other animals and must be gotten from a pure source. Maybe someday "true blood" can be made synthetically with it but that hasn't been successful yet.

addendum

With a little research into "fun blood facts", I'm sure we could find other things in Human Blood not in the blood of animals (other animals are affected by sunlight... but differently than Humans... so maybe it's something else?).

You could even use Facts like #12 - blood varies by population. IE: US is predominantly O positive and Japan is predominantly A positive... and groups in Tibet can survive in high mountains easier than others. Those differences will affect the humans available for the "monthly dietary needs" in different locales.

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A substance toxic to humans and vampires has contaminated the whole world. There is a drug that reduces the toxicity, but vampires can't metabolise it, however in the human blood there is the result of the metabolisation process which cannot be reproduced artificially, this is what vampires look for in the human blood.

Small plot variation, the drug might be the reason why some people turned to vampirism, some could easily metabolise it and some couldn't.

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Replenishing their own supply

Vampires have a circulatory system, but no way to produce their own blood. Their digestive process no longer digests blood, but instead pumps it directly into their own blood stream. They are immune to the effects of transfusion incompatibility in blood types, and don't need to replenish large amounts every day, just enough to cover the cell death of blood cells over the course a day or two. Consistent small intake is enough to keep a healthy supply.

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The vampiric red blood cells decay faster due to the higher metabolism. Unfortunately they do not build red blood cells any faster then normal humans. Due to this fact they get anemic quite fast and die if not ingesting/replenishing red blood cells from outside sources.

Animal blood only tides them over so much - it lengthens the period until they really need a human top-off. Sometimes they need the real deal and convert the ingested human blood into new bloodcells for themself.

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Accumulated mutations in a vampire's immune system cause a vicious reaction against significant amounts of non-human blood. Consumption of anywhere up to a fluid-ounce will probably cause a severe hives outbreak; a typical feeding amount could cause anaphylactic shock.

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