While training, AA2153 is shown an outdated video where everybody is cheerful and a stomach cell says it is always polite to introduce yourself. The naive red blood cell on his first day of work is quickly shown how horrible the body is. When he runs into the stomach cell, he introduces himself, only for the now jaded cell to say he doesn't care what his name is; he just wants his oxygen and the red blood cells all look the same to him anyway.
The Killer T Cell, driven mad from overwork and stimulation by cytokines, begins beating up the hair follicle cells, accusing them of being cancer cells. Eventually, the steroid robot restrains him, and AA2153 gets through to him that he can stop fighting. The Killer T Cell breaks down in Manly Tears over finally being given a break.
Frequently, the cells have to confront the reality that the body they're in isn't taking good care of itself. The horror of the futility of their work drives some of them into despair. When medical interventions help sustain it, they can't help but question if they've been rendered obsolete.
Red Blood Cells in the second body are becoming glycated due to excess glucose from diabetes, turning into dangerous, burned-out husks. QJ0076, the largest of the RBCs and able to absorb more glucose, has been eating the others' shares and eventually becomes glycated, but continues to devour their spare sugars as he burns while his friends plead with him to stop, because he'd rather die trying to help them than become a threat to other cells and the body.
QJ0076 also helped Glomerulus, who was severely overworked, by consuming excess glucose to lessen the amount she had to dispose of and assuring her that he'd be fine. They even had a bit of Ship Tease! After the body is saved by an insulin injection, we see a relieved Glomerulus hoping that the Red Blood Cell who helped her will visit soon, because she wants to thank him.
The second body that the red blood cell has been transfused to undergoes a whole lot of medical problems, presumably from working lots of overtime and getting very little sleep. Things come to a breaking point at chapter 31 when it attempts suicide by overdose. When it gets hospitalized again and gets resuscitated, it emotionally breaks down and releases a gush of tears of remorse. Several chapters later we learn it never made a full recovery as it lapses into depression.
After surviving one too many crises and losing one too many coworkers, SS1104 has had enough and calls out the brain cell commander, channeling several chapters' worth of frustration into a right hook.
K-9999 having his hopes crushed for failing his job application can ring a bit close to home for those who've struggled with real-life job interviews.
It's worse when you think about how he used to be so idealistic and eager to get to work and help the body get oxygen, even being one of the first friendly faces AA2153 encountered after arriving in the body. Being unable to fulfill his purpose as a result of the body's poor decisions destroys the poor kid.
Unlike the Cancer Cell from the original manga, K-9999 has no idea that he's cloned from a flawed cell or that his predecessor copies are cancerous; when he learns the truth, he's still trying to deny it when he suddenly mutates, impaling a Red Blood Cell. It's clear that he didn't intend to murder anyone, and he futilely tries to stop the other clones as they run rampant through the very lungs that K-9999 was so happy to work for. Even mutated and missing an arm, he still politely asks for oxygen because "If I can regain my strength... I can definitely contribute to this body..." When it looks like he's actually going to get the oxygen, he smiles gratefully and thanks the Red Blood Cell because now he'll be able to work for the body!
