What is Immune memory?

After victory, part of the clone army stands down into memory cells: long-lived specialists holding the antigen’s description. A second landing of the same invader meets a prepared corps — response in hours, invader cleared before symptoms: you are immune. One measles, never two; last winter’s strain unable to relight this year’s outbreak (Problem 69.1). Memory is why the same cold seems never to come twice — and why colds keep coming: their viruses mutate fast (Proposition 65.6) into descriptions the memory does not hold.