An immune system was a lifeform's defence against disease.
When Ashildr fell victim to the Black Death in 1348, her immune system coped and she got better. (TV: The Woman Who Lived)
Midge's sister died in the early 21st century when her immune system gave out, having been weakened by severe pollution. (PROSE: Cat's Cradle: Warhead)
Aaron Copley discovered that Martha Jones' immune system had mutated under the influence of radiation from her time travelling. (TV: Reset)
In 2009, Ianto Jones discovered that Freda didn't have any T cells, prompting Jack Harkness to wonder if she had no immune system. (AUDIO: Asylum)
In 2050, Alistair Gryffen discovered that Starkey had an alien immune system. This enabled him to resist the Oroborus' effects on time. (TV: Oroborus)
A Time Lord's immune system was seriously weakened when a regeneration took place. (PROSE: Interference - Book Two)
The Steggosians were wiped out by a plague that destroyed their immune systems. (PROSE: The Eyeless)