Cells at Work!

Cells at Work! is a manga that takes place in a human body. It's been compared to Osmosis Jones. That said, Cells at Work has much more accurate detail. It has an anime adaptation and spin-off mangas. The protagonists are a neutrophil (a type of white blood cell) and a red blood cell.

Tropes used in Cells at Work! include:

Adorably Precocious Child: The Platelets are clearly meant to invoke that feeling with their younger appearance but adult work, at least from a body's point of view.
Blood Is the New Black: White blood cells wear white (obviously) uniforms, which quickly become splattered with bacterial cytoplasm during germ invasions.
Darker and Edgier: Cells at Work! Code Black
Darkest Hour: Hypovolemic Shock. While the human body was in very real mortal peril during both Hypervolemic shock and heat stroke, the Hypovolemic Shock obviously had a much more dire tone. For example, basically nobody seemed to die during Heat Stroke, but in Hypovolemic Shock we see a bunch of red blood cells fall out of the human body and it's mentioned that more than millions are lost. Red Blood Cell almost freezes to death. Starting at the end of the briefing and ending with the Eucatastrophe all blood cells are basically in one of three camps: waging a battle against the apocalypse that's futility becomes more apparent as time goes on, Dead/missing, or given into crippling despair. With the initially large foremost camp rapidly moving into the second, and one gets into the third. The body's leadership doesn't know how to handle the situation. To demonstrate the devastating loss of personnel & life, the roads that represent blood vessels quickly become barren.
Eucatastrophe: Hypervolemic Shock and Heat Stroke. In the later, characters attribute the eucatastrophe to Secretion Gland Captain's Rain Dance.
Foreshadowing: The allergies that are suffered in the allergies arc are very low stakes. They are the kind that make someone feel sick and miserable, and are far from life threatening. Heck, even the microscopic single cell characters aren't in much danger. However the person that the body clearly took medicine to deal with it during the story. This reminds us that modern medicine is something that human bodies can receive these days, and also tells the audience that its use is fair game (at least in situations that would be noticed by people), which the author once again uses when the stakes are much higher and things look hopelessly grim.
Knight Templar: White Blood Cells; all of them, including the male protagonist. They show no mercy whatsoever to invading germs and toxins, brutally slaughtering them. Maybe he feels sympathy for them sometimes, like he did towards the Cedar Allergens, but never mercy. Killing these invaders is what White Blood Cells do.
Mood Whiplash: In the anime, things were not looking good at the end of part one of the "Hypovolemic Shock" plot. Then we get the usual cheerful end credits.
No Sense of Direction: AE3803; she's still a novice and tends to get lost while making her deliveries until U-1146 takes her under his wing.
Sweet Tooth: All red blood cells are fond of ice cream; although, in this case, "ice cream" is how the characters perceive glucose/dextrose, a substance that fuels nonfictional red blood cells.
Unstoppable Mailman: The red blood cells are depicted as energetic busybodies, always rushing to make their deliveries. While the same is true of AE3803, she's something of a novice and scatterbrain who tends to get lost a lot.
We Are as Mayflies: The reason the platelets look all so young is because they don't live long in Real Life: only twelve days at best.
You Are Number Six: Most of the main cast has designations that consist of letters and numbers, along with a label indicating what sort of cell he or she is. The male lead is referred to as Neutrophil or U-1146, while the female lead is Erythrocyte AE3803.