The report, published online in the Journal of Biological Chemistry, said the markers are believed to tell the female immune system that the sperm are not dangerous pathogens and should not be attacked by the woman's white blood cells during the reproductive process.

Researchers from Imperial College London and the University of Missouri said the marker also is found on some types of cancer cells, some bacterial cells, some parasitic worms and HIV infected white blood cells.

The report said these markers may allow such dangerous cells and pathogens to evade destruction by the human immune system, Imperial College said Wednesday in a news release.