A. Hyatt Verrill’s “The Exterminator” (1931).
The Exterminator is a fierce and savage creature living in a world
full of the strangest monstrosities, some of which are friends that he
peaceably ignores no matter how much they crowd him, and others of which are enemies that he lunges upon and devours whole. His
environment is a totally lightless tunnel of constantly flowing
liquid, where he and others like him joyfully wage a constant war
against invaders but are far more content when there are no invaders at all.
TWIST ENDING: The Exterminator is a white blood cell.
The most interesting thing about ‘The Exterminator’ is what it tells
us about the early days of science fiction, when this sort of plot
twist was considered acceptable. It’s not until I checked the
copyright data at the front of the anthology that I realized how old
this story actually is (Wikipedia says Verrill was born in _1871_ !)
— and it shows. It feels like rather like a nature documentary; you
can almost hear David Attenborough narrating it. I’m also intrigued by the role of chemotherapeutic agents at the end: this was published a good 11 years before the onset of the Antibiotic Age. In general, though, Verrill’s style isn’t strong enough to make the story worth it; I’m not sure anyone’s would be.