What attribute would allow certain blood types to be more malleable than others?

[Erebus]

There exists abilities that are specifically tuned to certain individuals or clans. One of these strange abilities is known as blood-bending, which is the power to turn an individuals' blood against them. Blood bending can be used in several ways to fulfill various purposes. A user can bend an individual's blood in order to control their movements and actions from a distance. It can be used to draw a victims blood through the pores in their skin in order to manufacture physical manifestations of objects instantaneously. This can be specifically used to form weapons in battle, such as swords, sickels, projectiles, etc. Finally, a user can draw out a persons' blood and absorb it into their own supply, healing wounds and restoring their life force.

While blood bending is a poerful technique, its effectiveness can vary between the different blood types. Certain blood types are blood types are barely malleable, and would remain mostly unaffected by a skilled blood bender. Others are moderately effective, with a user being able to control their movements to some extent, and create brittle and easily shatterable weapons from them. Some can be very affected by a blood bender, who will be able to create firm and powerful weapons from their blood, use their blood to heal themselves, and take full control of the victim's body.

I need to find some specific distinction between blood types that would justify making some more receptive to being controlled by another's influence than others. What attribute would I be able to use for this power?

 

[Crescent]

I'll be honest, I don't know much about the science of blood types, but I can think of a justification from the magic side of things.

You could make it so that individuals can control blood of their blood type easiest and have increasingly more different blood types be increasingly harder to bend, so for example, an AB positive person is best at bending AB positive blood, and worst at bending O negative blood.
This could be justified by saying that the magic is an extension of the user, and users with different blood types simply aren't as compatible due to the magic making its home in the blood of the user, the magic bonding itself to the material it makes its home and remembering it in a sense.

If you wanted to, storing magic over any material being housed in said material would make sense here, say fire magic living in fire, or water magic living in water, with magic merely adopting the form of the material its housed in, but that's completely up to you and dependent on the existence of other magic types.
That's the idea though, magic has memory, hope it helps.

 

[Rosemary Tea]

By blood types, do you mean ABO blood types? Or are you interpreting that more broadly?

ABO blood typing determines compatibility for transfusions. It also has some correlation with vulnerability or lack thereof to certain diseases. Mostly, these are diseases that aren't current threats, but Covid-19 has been found to be less of a threat the fewer of those antigens you have. Type O and rhesus negative blood are at the lowest risk for Covid, with O negative being lowest risk of all. The highest was B positive, with type AB people having a higher risk than type A but lower than type B. So, apparently, antigen B and the rhesus factor raise the risk.

If you want to stick with ABO blood typing, one type can be the most vulnerable to blood bending and one can be the least. If you make it a broader thing, blood types could be determined some other way (is there perhaps a dragon type, a unicorn type, a griffin type?) and pick whichever you think would be the most vulnerable.

 

[K.S. Crooks]

Perhaps the level of iron in a person's blood or even some people have other metals instead of iron (chromium, cobalt, nickel, copper). Perhaps there's an alternate or addition to the ABO Rh + Rh- we have. A third blood type could denote blood bending ability.

 

[Queshire]

I'll be honest, I don't know much about the science of blood types, but I can think of a justification from the magic side of things.

You could make it so that individuals can control blood of their blood type easiest and have increasingly more different blood types be increasingly harder to bend, so for example, an AB positive person is best at bending AB positive blood, and worst at bending O negative blood.
This could be justified by saying that the magic is an extension of the user, and users with different blood types simply aren't as compatible due to the magic making its home in the blood of the user, the magic bonding itself to the material it makes its home and remembering it in a sense.

If you wanted to, storing magic over any material being housed in said material would make sense here, say fire magic living in fire, or water magic living in water, with magic merely adopting the form of the material its housed in, but that's completely up to you and dependent on the existence of other magic types.
That's the idea though, magic has memory, hope it helps.

They went this route with the character Stain in the manga My Hero Academia, but I wasn't that wild about it. Readers generally don’t know or care about a character's blood type which can make it feel a bit arbitrary and techy. I mean, it fits with the pseudo-science thing the My Hero Academia has going for its powers and that Stain's dangerous as a person. Not because he hit the jackpot for powers.