The quirks of My Hero Academia- Katsuma Shimano

The title alone isn’t enough of a warning, here is a second one.  Yes, there will be spoilers for the My Hero Academia movie.  First off, the movie is awesome and if you are a fan of My Hero Academia, you should check it out and hopefully before it leaves the theater.  Over the course of the movie we are introduced to a young boy by the name of Katsuma Shimano, and more importantly, his quirk cell activation, which the villain Nine needs to stabilize his weak body. 
Over the course of the film we don’t learn all that much about the quirk beyond that it does something to the cells of the body, can heal people, and possibly make them stronger.  The down side is that it drains an individual’s stamina and can only be used on people with compatible blood types.  While I doubt that we will learn much more about Katsuma or his cell activation quirk, if we do, I hope that Horikoshi does not go the reality warping route that he did with Eri.  (FYI- which completely invalidated my theory on how her quirk actually worked.)  Thus, assuming Katsuma is not a reality warper, how does his quirk work?

Cell activation

Cell activation
The first thing that comes to mind when I think cell activation is the activation of T and B cells during the body’s specific immune response to a pathogen.  While I could say more about T and B cell activation, it is fairly obvious that the cell activation quirk has little to do with activating the immune system.  Not that being able to supercharge the immune system wouldn’t be a good thing, it just wouldn’t do much to help Bakugo and his broken ribs.  We can assume that they are broken because Bakugo says that they are, and anyone taking a beating like he did probably would have broken ribs.  Normally it would take weeks for broken ribs to heal, not a few hours like we see in the movie.  Thus, this begs the question of what does Katsuma’s cell activation quirk actually do?

Cell Differentiation

Excluding a reality warping explanation, the most logical or scientific explanation for the cell activation quirk is actually cell differentiation.  In a nutshell cell differentiation is the process by which a cell changes from one cell type to another cell type.
cell differentiation
In other words, cell differentiation is how you went from a single fertilized egg to the you who is reading this article.  The process of cell differentiation is governed by differential gene expression, and this is what the cell activation quirk really controls.

Differential Gene Expression

Scientists currently estimate that humans have somewhere in the range of 20,000 to 25,000 genes, and just like how you do not open every program on your computer or phone, the human body does not turn on all of the genes at the same time.  The pattern of which genes are on or off determines if a cell is a muscle cell, nerve cell, or some other cell type.

What does all of this have to do with healing?

Surprisingly, a lot.  As we all know healing takes time.  Time for the body to recognize that it has been injured, time for the body to begin responding to the injury, and time for the healing process.  This process involves a number of different cellular signals activating or shutting off different genes, driving the healing process.  The healing process can include cells releasing or not releasing certain compounds, cells moving into different areas, and cells dividing.

Healing via the cell activation quirk

Katsuma’s cell activation quirk doesn’t really activate cells, but activates genes.  By changing the gene expression of cells he can completely change the cells activity.  Depending on which cells he changes, and how he changes them, Katsuma can dramatically speed up healing.  In the case of Bakugo’s ribs, Katsuma probably changed the following cells:
Osteocytes (mature bone cells) to Osteoblasts (cells that form new bone)
Osteoclasts (Cells that breakdown bone) to Osteoblasts
White blood cells to Osteoblasts
White blood cells to Chondrocytes (cartilage cells)
If more cells are needed, Katsuma could go so far as turn the cells of other organs into stem cells, move them to the injury and then turn the stem cells into the needed cell type.  In the case seen in the anime I would go with skin cells, or the cells of the intestines.
(FYI- Red Blood cells do not have a nucleus or DNA, so there are no genes for Katsuma to modify)
By increasing the number of osteoblasts and chondrocytes, the speed at which the bone heals would be increased.  By how much I can’t speculate given the lack of information we’re given on Baguko’s injury.  Additionally, if Katsuma can change cells from one type to another, he can change the behavior of cells in a way that can enhance the healing process.  Such changes could include reducing inflammatory response, which reduces inflammation.   He could also cause Bakugo’s body to release more endorphins, which are natural pain killers.  (FYI- Endorphins are released during exercise, and by foods like chocolate.)

Strengthening the body

The second aspect of the cell activation quirk is strengthening the body, which is referenced by Midoriya after recovering from being beaten to a pulp by Nine.  As it happens, cell differentiation can also explain Midoriya’s feeling stronger.  The first and most obvious answer is that Katsuma made Midoriya’s muscle cells produce more actin and myosin, increasing their strength, which is normally seen as a result of strength training.
muscle fiber
The actin and myosin are the proteins that cause muscles to contract and in layman’s terms, the more you have the stronger you are.  In addition to an increase in muscle fibers Katsuma could also increase the number of mitochondria in muscle cells.  The increased mitochondria increase energy supply for the muscles, improving strength and stamina.  This normally occurs as a result of aerobic training.
Next up is the red blood cells, specifically the amount.  It is possible that Katsuma directed Midoriya’s bone marrow to make more red blood cells, improving his ability to utilize oxygen, improving stamina.  An increased amount of red blood cells is seen in individuals who live at higher altitudes.
The final adaptation is the modification of bone tissue.  For those of you who may not know, bone is not solid and is actually hollow.
 The direction of the solid bone, thickness of the bone, and shape of the hollow areas is dependent on the forces placed on the bone.  Thus, even identical twins, whose only difference is what sports they participate in, will have different bone patterns.  It is possible that Kastuma by changing the behavior of bone cells could make the bone stronger.

Future possibilities

Based on his appearance Katsuma is only 6-8 years old and he healed a comatose Bakugo and Midoriya to the point where they could fight Nine again and eventually defeat him.  He did all of this without any quirk training and more importantly, no knowledge of human anatomy or physiology.  Jut imagine what a trained Katsuma with knowledge of human anatomy and physiology could do.  All Might’s missing organs, no problem as Katsuma can basically order his body to make new ones.  In the case of Yuga Aoyama, Katsuma’s abilities could help strengthen his body reducing the strain his navel laser puts on him.  Of course, this would make him a prime target for All for One, but it also brings up an interesting situation.
All for One needs physical contact to drain quirks, and Katsuma needs physical contact for his quirk to work.  Now I bet all of you are thinking then All for One will just drain Katsuma’s quirk, but you are forgetting something, quirks are genetic, and the cell activation quirk changes the behavior of genes.  The moment All for One grabs Katsuma, he can use his quirk to shut down All for One’s quirk.  Katsuma can’t steal quirks, but he could potentially with training turn quirks on and off just like he does with genes.  And unlike Eraser head the deactivation would be permanent or at least last a good long while.


I hope you all enjoyed a little more science of quirks from My Hero Academia.  I know I didn’t go into as much detail on this one as I have on other quirks, but cell differentiation and differential gene expression gets complicated pretty quick.  If I missed any quirks that you would like to see, let me know.
The quirks of My Hero Academia- Katsuma Shimano The quirks of My Hero Academia- Katsuma Shimano Reviewed by Author on April 29, 2020 Rating: 5

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