- Though most of Red Blood Cell's terrified screams are played for laughs, the part at the end of Episode 1, when the Pneumococcus pounces on her, has her screaming in a way that's less funny and more upsetting. She genuinely sounds hopeless and frightened in that moment.
- It's kind of sad seeing Erythroblast saying good-bye to the Myelocyte who saved her life, waving and calling out to him as they part ways, with her expecting that they would never meet again. Fortunately, it isn't their last time.
- Erythroblast seeing the Red Blood Cells working, and wishing she could be like them. Later, when a Pseudomonas is trying to kill her, she defiantly shouts to herself that she refuses to die now as she'll be a great Red Blood Cell someday.
- Red Blood Cell going out of her way to check on an injured Neutrophil, during the scrape wound incident. The way she thanks Neutrophil for all he's done and the way he replies how he's only doing his job is a mix of this and Heartwarming.
- Neutrophil showing a sense of mourning regret toward some of his enemies:
- He pauses momentarily at the Influenza zombies, lamenting that they were once normal cells that were infected by viruses.
- Later, he regretfully bows after killing the Cedar Allergen, understanding that while it was a threat to the cells it ultimately was a Non-Malicious Monster.
- Seeing the other cells being bullies toward Eosinophil, whose job is to save their own lives, yet are complete Ungrateful Bastards toward her, even as she willingly struggles to fight off germs to defend them, despite it not being her actual purpose. The way she starts calling herself weak and useless really hits hard.
- Though quickly and hilariously interrupted by Red Blood Cell and the Steroid, there is something rather tragic about Neutrophil and Memory Cell's conversation about the allergy, and how there was no way to stop the reaction. Everyone did their jobs as they were supposed to, and yet it became a disaster anyway. It's a surprisingly existential moment pondering the brief, futile life of a cell.
- Naive T Cell despairing about his inability to fight, with him crying about how he'll never live up to the standards of his teammates.
- The aftermath of the Steroid's rampage in episode 5 is played with utter seriousness, as opposed to the goofy punchlines of other episodes. You can really feel the sense of tragedy that was a necessary evil to thwart a worse scenario.
- Cancer Cell's entire story arc, with him being a tragic villain. He never chose to become what he is, or even exist in the first place, and finds all life cruel and pointless, thus seeking to destroy the whole body along with him if he isn't allowed to live.
- After NK mortally wounds him, Neutrophil jumps in to finish him off, but hesitates. He doesn't kill him right away, he lets Cancer Cell say a few things and hears him out, in a surprisingly sympathetic moment from the normally merciless Neutrophil.
- Cancer Cell's lamentation that he fought and lost and will vanish without a trace, made worse when the scene cuts to the other cells in the distance celebrating his demise.
- In episode 8, Neutrophil's heartbroken look after Killer T attacks him for siding with a red blood cell, and the way he actually appears sad and frightened as Killer T yells at him for being too soft.
- And later, the reveal that Killer T actually feels envious of the other cells having friends, but is simply too manly to admit it. He even seems almost on the verge of tears as he says to himself that "he isn't envious". Poor guy's in denial.
- Seeing Neutrophil and his squad get beat up by the giant Staphylococcus, and Red Blood Cell being terrified for him and screaming for him to escape. You can see how much she's come to care for him.
- In chapter 11, we see an Ordinary Cell befriend a Rhinovirus-infected cell, but their relationship quickly ended when his newfound friend tried to infect him and his copy, and was promptly killed by the Immune Cells. May double as a Fridge Horror if one considers how much sentience an infected cell still has, and how much of Rhinovirus's antics in the beginning may have been driven by the former cell's own desires, before he finally succumbed to the virus's corruption.
Tear Jerker / Cells at Work!