Eri- the Anti-Quirk
The cute mysterious waif of My Hero Accademia, Eri, is much more than she appears, and is a major plot element during the current Internship Arc. Not much is known about Eri’s past or even her quirk, but what is known is that her blood is being used to make anti-quirk bullets, which shut down a person’s ability to use their quirk for an unspecified period of time. As if this wasn’t dangerous enough, the bullets are later upgraded to the point where they can permanently remove a person’s quirk.
The ability to shut down and/or remove a person’s ability to use their quirk is not unique to Eri in My Hero Hero Academia as two other characters can remove quirks. The first character that can impair quirks is Eraserhead. If we think of quirks like another system of the body, then Eraserhead’s quirk is able to shut down a person’s quirk as long as he is looking at his target. Sensei is the second character with the ability to stop/remove quirks. Sensei’s quirk “all-for-one,” allows Sensei to completely remove a person’s quirk. Sensei can then keep the quirk for himself or transfer it to another. In essence losing a quirk would be like losing a limb or special sense and is probably quite painful and debilitating. Also, gaining a quirk could also be painful and difficult, like suddenly adding a new system to the body. While Midoriya did not have a lot of pain in gaining his quirk, we do see that he is still struggling to control it.
We do not see just how Sensei uses all-for-one to remove quirks from heroes, and the key note here is remove. He is able to remove the quirk and transfer it to himself, while leaving his victim alive, but quirkless. I would speculate that all-for-one is using some sort of plasmid that is able to enter the victim’s cells and cut out the quirk DNA and bring it back to Sensei. It would share some similarities to one-for-all that I cover here:
On the other hand, Eri’s blood and probably her quirk stuns the quirk system of a person’s body, temporarily preventing it from working. It is this unique aspect of Eri’s body that Chisaki weaponizes in the form of anti-quirk bullets containing Eri’s blood. Chisaki is able to upgrade the bullets to the point where they can destroy quirks with his new and improved anti-quirk bullets. In either case the quirk is not shut down/destroyed instantaneously like what is seen with Eraserhead, so there is most likely a real physiological system acting to destroy the quirk. I can think of several biological mechanisms by which her blood can destroy quirks: viral, transfusion reaction, and toxins.
Eri’s blood could contain a cell or virus that is able to enter the cells of the victim and affect the DNA related to the individual’s quirk. This would be very similar to all-for-one, except the DNA is destroyed instead of removed. I don’t think this is the case as it would need a not insignificant amount of time to work and Lemillion lost his quirk in a very short period of time after being shot, much faster than a virus would be able to work.
The next mechanism that came to mind is a transfusion reaction, which is what happens when you receive the wrong blood type. There are 4 blood types and in each blood type the red blood cells have a particular protein on their cells identifying them, and the blood plasma contains antibodies for the other marker. The chart below will help simplify things.
This means that if a patient with type A blood receives type B blood, the antibodies in the type A blood will attack the type B red blood cells, and the antibodies in the donated type B blood will attack the patient’s type A red blood cells. Needless to say, this is going to destroy the blood cells and it’s not a good thing. This reaction is called agglutination and you can think of it like the blood clotting inside of your blood vessels.
Obviously, this is not going to be very good for your health and it is why hospitals are so careful when giving patients blood, in order to be sure the correct type was given. In most cases the degree of agglutination is linked to the amount of the incorrect blood type that is given. This is relevant to Eri’s quirk because agglutination reactions happen rapidly and if her blood has some factor that is incompatible with a quirk user blood, a reaction could occur causing the destruction of the quirk system, until the body is able to heal.
You might be thinking that this is a perfect explanation and the is problem solved; well, not exactly, since a bullet-sized capsule would not deliver very much blood. The small amount of blood would only result in a small quirk/anti-quirk reaction. This means that while you might be able to cause a person to lose their quirk in one part of the body, it would not affect the entire body. This means the quirk bullets would be more or less dangerous for certain individuals. For example, Ida whose quirk is located in his legs, an arm shot does nothing while a leg shot would be extremely problematic. On the flip side, it wouldn’t matter where you hit Mina, who releases acid from her skin, but it would only nullify a small part of her body.
Also, because only a small amount of blood is used for each bullet, it is possible for someone shot with anti-quirk bullets to recover the use of their quirk as the body heals. This is what happens to the first hero shot with the anti-quirk bullets as he is able to recover the use of his quirk over time. However, Chisaki is able to upgrade the bullets to the point where they can destroy a person’s ability to use their quirk permanently, which is what happens to Lemillion, and there is a potential explanation for this.
DIC, or disseminated intravascular coagulation, occurs when all of the blood in the smaller blood vessels of the body begins to clot. This is a very serious condition that can lead to organ failure and death. It has a number of causes, including transfusion reactions and allergic reactions. Before I continue with DIC and its connection to the anti-quirk bullets, let’s review Eri’s quirk for a moment. Let’s assume that Eri’s blood contains a factor that attacks the quirk system of another user’s body, so she is like type AB blood, so she can receive blood from anyone without having to worry about her quirk system being damaged. That means, however, that her blood will attack the quirk system of anyone that she gives blood to. Clearly this is a problem for other quirk users, but nothing that can’t be recovered from.
For Chisaki to upgrade the anti-quirk bullets to the point where they permanently remove a hero’s quirk he would have to add something that would take Eri’s quirk attacking the quirk system and make it cause something like DIC through a person’s quirk system. It’s not as farfetched as it sounds, since Eri’s blood is already damaging the quirk system of a hero’s body. In the real word DIC can lead to organ failure, as blood clots inside of them. The enhanced bullets are likely causing a DIC-like reaction inside the quirk system, destroying it.
There is only one problem with this explanation and that is Eri’s blood itself. We have seen quirks based on blood types before, like Stain’s bloodcurdle quirk, which varied in effectiveness based on the blood type of the affected person. It is entirely possible that there are people out there with a similar blood type/quirk type to Eri and they would not be affected by her unique quirk. Secondly, I would be curious to see if it could do anything to Midoriya, since he was quirkless at birth and gained a quirk later. His biology might be just different enough that it could have less of an effect on him.
The role of the immune system is to protect the body from foreign substances that enter it. These substances can include bacteria, fungi, viruses, parasites, toxins, and other substances. The immune system locates these invaders and destroys them before they can harm the rest of the body. As you can imagine this is a very important system, and without it we wouldn’t live very long, as our bodies are bombarded daily by a multitude of harmful and harmless substances. The trick, of course, is having the immune system attack the harmful substances and leave the harmless ones alone. Most of the time this is what happens, but occasionally the immune system gets confused and attacks a harmless substance as if it were a harmful one. This immune system mistake is known as an allergy, and 30% of American adults and 40% of American children have one.
I would hazard a guess that most of you who are reading this either have an allergy or know someone who does. You might also be thinking that allergies are mostly harmless or just annoying. You are correct, but there are some allergies that can be fatal when the body has a gross overreaction to an otherwise harmless substance. Some examples of allergies that can be severe include bee stings, shellfish, and nuts. This reaction is called anaphylaxis and it causes the following symptoms (and others):
Red itchy skin
Abnormal heart rate
The airways can constrict to the point that a person with anaphylaxis can no longer breathe, while the abnormal heart rate can cause a heart attack in some cases. But what does this have to do with Eri’s quirk stopping abilities? In a word, lots. Eri’s blood might contain a substance that causes an allergic reaction when it is introduced into a quirk users body. This allergic reaction could cause the affected individual’s quirk system to seize up and stop functioning, not unlike the respiratory and/or cardiovascular system seizing up in a severe allergic reaction.
I like how simple and clean this explanation is because an allergic reaction happens rapidly, just like the quirk loss when heroes are struck with anti-quirk bullets. It also means that Chisaki needs Eri to continually make the bullets, because it is a factor in her blood that he can’t replicate, as we see in the manga. The allergy theory for Eri’s quirk also explains how her blood only causes a temporary loss in a hero’s ability to use their quirk.
The problem with the allergy theory is that I cannot think of a way for Chisaki to upgrade it to the point where it would destroy the quirk without harming the rest of the body. The second problem is that not everyone will overreact to Eri’s blood, just like not everyone overreacts to substances like peanut butter or shellfish (two of my favorites). Again, I would look to Midoriya as an example of a person who might not be affected due to the unique nature of the way he gained his quirk. A final issue is that if the anti-quirk bullets are working by causing some sort of allergic reaction, the heroes would only have to carry epipens, which are a very effective treatment of anaphylaxis. The epipen is a device that injects epinephrine, which counteracts the negative effects of extreme allergic reactions. The epipen works quickly, so it could prevent the hero from losing his or her quirk during the fight, but it would have to be reapplied after 5-15min depending on the severity of the allergic reaction, because it is short acting.
The last possible explanation for Eri’s quirk-stopping ability is that her blood contains a toxin that targets the quirk system of individuals who come into contact with her blood. There are a number of real world animals that use toxins in a variety of ways including spiders, scorpions, frogs, and snakes to name a few. The question now is, if Eri’s quirk is making a quirk-paralyzing toxin, how is it that she is still a captive of Chisaki? First off, her quirk only makes another person’s quirk stop working, and she is a little girl, so a quirkless Chisaki can still overpower her. The second is that it matters how her anti-quirk toxin is released. Some animals are only toxic if you touch them (poison dart frogs), others need to inject their toxin into the body (like snakes), and the last group has the toxin inside their body, so they need to be eaten for the toxin to take effect (like monarch butterflies). I would hazard a guess that Eri falls into the final category, where her blood is toxic, but she lacks the ability to expel it from her body. This means that the only way Eri’s toxic blood can affect someone is if it’s injected inside them.
The second issue with toxins is the type of toxin. There are a variety of different types of toxins, but they can all be put into two basic categories, neurotoxins and hemotoxins. Neurotoxins tend to be fast-acting as they affect the nervous system, while hemotoxins destroy red blood cells. In Eri’s case, her blood probably contains a toxin that targets the quirk system and paralyses it. This is not unlike the real-world drugs succinyl choline and curare, which cause muscle paralysis and are used in anesthesia. Succinyl choline causes muscle paralysis and is used to relax the muscles of the throat during endotracheal intubation, or sticking a tube down your throat so you can still breathe.
I don’t think it is a big stretch to think that Eri is producing a toxin that can specifically target quirks. The toxins would work fast, but not instantaneously, fitting the time frame that is seen in the anime. The paralysis would also not be permanent, allowing for heroes shot with the non-upgraded quirk bullets to recover. The toxin I like best for this is something called tetrodotoxin (a neurotoxin), which can cause paralysis among its many symptoms. Tetrodotoxin is found in puffer fish, and when a person eats puffer fish they are ingesting a tiny amount of toxin, just enough to cause a tingling sensation on the lips and tongue, but not enough to kill them. In higher amounts, it is extremely dangerous.
This is where things become more interesting for Eri, because the puffer fish does not produce its own toxin; it’s made by symbiotic bacteria that live in the puffer fish. The quirks of My Hero Academia are not limited to humans, as some animals have been known to manifest quirks. One such example is Principal Nedzu, who is some sort of furry animal with a quirk that makes him extremely intelligent. In Eri’s case her quirk is not actually making an anti-quirk toxin, but the ability to have bacteria live in a symbiotic relationship with her. Specifically, bacteria that have a quirk causing them to create an anti-quirk toxin.
This would explain why Chisaki still needs Eri, because he hasn’t figured out how to grow the bacteria outside of Eri’s body. It also explains how he was able to upgrade the quirk-paralyzing bullets to quirk-destroying bullets. He collects the toxin from Eri’s blood and then concentrates it to the point that it can destroy the quirk system instead of paralyzing it. Not unlike having a small amount of puffer fish is ok, but having a lot will kill you.
Over the course of this post, I have given you a variety of theories on how Eri’s anti-quirk abilities could potentially work, but which one is the best one? If I have to put money on how Eri’s quirk worked, I would go with the toxin explanation. It is the only theory that fits all of the information given in the manga and is the simplest of the four explanations. In philosophy, there is a principle known as Occam’s razor, which means the explanation with the least assumptions is the most correct one. I like to explain it to my students as KISS, keep it simple, stupid. The toxin theory explains Eri’s quirk, why chisaki still needs Eri after making the bullets, and explains an easy pathway to upgrade the bullets. There is also the benefit of no possible treatment or situations where the bullets might not work.
Update- It turns out I was completely wrong on Eri’s quirk and she is more of a reality warper.