Immune system

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)

In 2008, 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)