If 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?
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?
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.
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.
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.)