LOS ANGELES: As recently as 2008, scientists thought that Neanderthals and modern humans had never mated.

Then, last year, they said that the two species had - but that the few Neanderthal genes that survived in modern human DNA were not functional.

Now researchers believe that important versions of immune system genes in modern humans appear to have been passed down by archaic relatives, including Neanderthals, after all.

Indeed, DNA inherited from Neanderthals and newly discovered hominids, dubbed the Denisovans, has contributed to types of immune genes still present today among populations in Europe, Asia and Oceania.

This DNA has had ''a very profound functional impact in the immune systems of modern humans'', the study's first author, Laurent Abi-Rached, a postdoctoral researcher at Stanford University's school of medicine, said.

Neanderthals were stocky hunter-gatherers who populated Europe and parts of Asia until about 30,000 years ago.

Dr Abi-Rached and co-authors focused on a small set of genes on chromosome six, known as the human leukocyte antigen (HLA) class I genes.

HLA genes carry instructions for making HLA proteins, which help the immune system spot evidence of problems in cells so that it can wipe out abnormalities to fight disease.

From the analysis, the scientists estimated that more than half of the genetic variants in one HLA gene in Europeans could be traced back to Neanderthal or Denisovan DNA.

Los Angeles Times