Dr Hilary Jones explained on Daybreak how Mark Cahill's hand transplant was a world first.
He said hand transplants have been done since the 1960s, however this is the first time that the hand has been removed and the donor organ has been transplanted on at the same time.
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The Labour leader did not say whether he would maintain Britain's nuclear deterrent programme.
Ukip leader Paul Nuttall also said he would want to get rid of Sharia courts.
Over 40,000 people picked up their packs to take part in the annual marathon in the capital.