In 1989, Charles Janeway, a scientist at Yale University, had an epiphany that would revolutionise immunology. For 50 years, immunologists had subscribed to the dogma that vaccines worked by training the body to recognise molecules that were foreign to the body – “non-self” in immunological jargon. The usual way of doing this was to use vaccines to expose people to a dead or harmless version of a microbe, prompting the activation of antibodies that would be ready to swamp the germ should they encounter the alien entity a second time.
But there were exceptions to the rule: sometimes, proteins separated from originating germs proved ineffective as vaccines; at other times, vaccines required the addition of an adjuvant, such as aluminium, to kickstart an immune response and no one could explain why. What if, wondered Janeway, the presence of something that had never been in your body before was not sufficient to trigger an immune reaction? What if a second signal was required?
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