Amazingly, this seems to be true. He appears on the clip below (dated 8/5/09), from just after 3 minutes in, where he analyses the blood of presenter Michael Mosley in order to establish whether a two week cold shower regime has benefited Mosley’s immune system.
According to Denton, this would be demonstrated by an increase in the numbers and activity of white blood cells. He assesses this by simply looking at a tiny sample of blood under his microscope before and after the two weeks. He concludes that Mosley’s blood sample following the shower regime shows ‘more activity’ of white blood cells, though Mosley points out that ‘a real boost to my immune system would have also seen their numbers rise’.
EDIT 25/10/11 The clip has been taken off YouTube but is still visible on the Live Blood Test site here. This may be a result of concerns raised to The One Show about Denton using his TV appearance to enhance his credibility.
EDIT 09/11/12 The clip now seems to have been taken down from the Live Blood Test site.
This is of course not an accurate method of doing a white blood cell (WBC) count.
Furthermore, the immune system is far more complex than Denton implies or probably realises. I’m not sure exactly what is meant by a ‘boost’ to the immune system but an obvious short term increase in the WBC count, rather than indicating such a thing, would typically be a sign of infection.