The Pneumococci are pretty terrifying when they first appear, rising from the floor, their tentacles waving menacingly and their eyes glowing as they gloat about taking over the body. And while they're easily killed by the Neutrophils, the fact that one survived and was hitchiking around the body in Red Blood Cell's cargo is Paranoia Fuel to the cells in-universe...
Staphylococcus aureus, basically queen of the germs: she knew how to evade the Neutrophils because she mutated into a smarter, deadlier strain after clashing with them before, she set the stage for other opportunistic bacteria to Zerg Rush a wound, and she's a perfectly normal part of the skin's microbiome. But you probably already heard of her before from smash hits like staph infections, bacterial meningitis, and MRSA.
The Influenza infection is portrayed as a straight-up Zombie Apocalypse, with some horrific shots of innocent cells being infected with the virus and screaming in terror as they are quickly and painfully turned... and with no way back, forcing the immune cells to kill them to avoid them infecting more.
The scrape wound. To us, a little scratch that's no big deal. To the cells, a massive sinkhole as wide as a city block, with cells that fall through disappearing forever. To make things worse, there are also evil, alien-looking bacteria coming through the wound to wreak havoc upon the body, making the scrape wound basically a portal to hell itself spawning monsters...
They're the good guys, but the murderous rage of the Neutrophils can be quite terrifying to behold. Even the other cells are quite shaken to see the bloodthirsty heroes gruesomely eviscerating bacteria, then eating them.
Remember those tiny parasitic worms you see in documentaries? From the perspective of the body cells, they are gigantic monsters that destroy everything in sight.
The Pseudomonas bacterium of Episode 6 who invades some bone marrow doubling as a daycare centre for little kid cells; he doesn't need oxygen or a steady supply of nutrients, so he can go anywhere at any time. He just picked the marrow so he could torture helpless blastocytes for fun before killing them.
Cancer Cell. A grotesque, twisted, mutant version of an Ordinary Cell, now a dangerous and uncontrollable psychopath able to masquerade as one of the Ordinary Cells so he can secretly fill the body with cancerous growths until it's too late for the immune system to save the day.
The original Cancer Cell is mutated, but at least he's still obviously humanoid. The clones he creates, however, are hideously half-formed, ranging from white-haired humanoids with empty eye sockets to gruesome masses of living flesh with multiple twisted limbs and a single huge mouth. If enough of them clump up together, they create tumours with random teeth, heads, faces, and limbs that sprawl over everything in their path.
His backstory is even worse: he had been a blastocyte with genetic mutations, so he started off already scared of being hunted down and viciously beaten to death by authoritarian police for the sole crime of being born, even though he's still a part of the body just like everyone else. ... A part of the body that will destroy it if left alive, which Cancer Cell fully embraces because if he can't live as part of the body, no-onecan.
The hypovolemic shock incident. Imagine the aforementioned scrape, but worse in every way. Millions of erythrocytes are MIA and/or killed, ordinary cells slowly choke to death because they can neither receive oxygen nor get rid of CO2 without those red blood cells, and leukocytes are overwhelmed with wave after wave of opportunistic infections trying to raid the body. Things end well thanks to a last-minute blood transfusion, but it's still the only time in the series that the body and their cells have been close to death, something that not even Cancer Cell managed to do.
The anime adaptation conveys this futility dauntingly well. After the emergency broadcast, we see everyone, from the Red Blood Cells to the Killer T-Cells to the Platelets, reluctantly steel themselves for what needs to be done as they prepare to face it. The next two scenes are of AE-3803, her kouhai, and U-1146 combing through the wounded area. The music is gone. A bustling city is completely deserted. Nobody is here. And in the background, as U-1146 stares on in stunned horror, the heartbeat gets louder and stronger, but slower.
The horror gets another layer once it's revealed that the wound which caused the hypovolemic shock get identified as a head injury. The organ that probably got cut off from blood supply? The human's brain.
We also actually "see" the injury when some poor red blood cells have to traverse it via a thin chain on a tiny ledge, and the contrast with the scratch earlier is mind-boggling. The latter was a block-sized hole in the ground. This, however, is a vast horizon-to-horizon canyon swallowing up the landscape. There was no way this person could've survived without medical intervention.
Cells at Work's depiction of heat stroke is surprisingly awful due to the cells having no ability to fix it by themselves. Fluid reservoirs dried up a long time ago and secretion cells no longer have backups to dip into, all natural methods for temp regulation are either not doing enough to help or failing altogether, a germ gleefully invades after the body passes out, and cells suffocate and crowd in the rising heat, comforting each other as they pray for a miracle. Compared to malignant cancer, traumatic head injuries, and COVID-19, heat stroke is pedestrian, but that doesn't mean it isn't hell on your body. Just... drink some water, okay?
A stampede of red blood cells getting trapped by the one-way door, pushed up against each other, banging on the windows and screaming for help, while bloodthirsty killers close the gap on the other end. There is a very specific reason why staying calm during an emergency evacuation gets emphasized.
The COVID infection. The virus by itself isn't much in combat... but it is incredibly sneaky, irritating, and extremely hard to put down without antibodies causing a cytokine storm. Then the immune cells start attacking everyone in sight. As if that wasn't enough, the damage they cause results in damage to the blood vessels, prompting the platelets to accidentally cause repeated thrombosisnote an excessive blood clot that blocks a blood vessel, and they even damage the pulmonary alveoli, almost killing the body from a lack of oxygen. Inhibitory cytokines are released just in time to stop the White Cells from destroying the body by accident... And they're appalled when they see the damage they caused.
