Sperm could provide a vital clue to how diseases like cancer and HIV spread through the body, a study suggests.
Sperm cells are "invisible" to the female immune system
UK researchers have identified markers on the surface of human sperm which prevent them being attacked by the female immune system.
The markers are also found on cancer cells and HIV-infected blood cells and may help the diseases to take hold.
The study, by researchers at Imperial College London, is published in the Journal of Biological Chemistry.
Experts say little is known about how sperm dodge immune system barriers but there are likely to be many mechanisms.
The female reproductive tract is a "hostile environment" for foreign cells which are readily attacked by the immune system but sperm move through apparently undetected.
Sperm are also protected from harm in the testes from the male's own immune system.
The Imperial College team says it has found specific sugar molecules on the surface of sperm which seem to be responsible for evading the immune response.
These glycoproteins are universally recognised by all human immune systems, regardless of the individual, say the researchers.
This differs from other cells which carry chemical markers recognised by the individual's own immune system, but may come under attack if placed in a foreign body - for example in the case of organ transplant.
The glycoproteins found on sperm are also present on some types of cancer cell, some bacterial cells, parasitic worms and HIV-infected white blood cells.
The researchers are now planning to look at how exactly the glycoproteins trick the immune system into believing they are harmless.
Dr Stuart Haslam, a lecturer in molecular biosciences, said in the case of sperm, it is in the human's benefit not to recognise it as dangerous.
"But it looks like some cancers and HIV-infected cells have hijacked this lack of recognition."
His colleague Professor Anne Dell added: "If aggressive cancers and pathogens are using the same system of universally-recognisable markers to trick the immune system into thinking they're harmless, we need to work out exactly how this interaction works.
"Understanding how these markers work at a basic biological and chemical level could lead to new ways to treat or prevent cancers and other diseases."
Professor Richard Sharpe, from the Medical Research Council Human Reproductive Sciences Unit, said the research was extremely interesting.
"The whole process of immunity in the male reproductive system is something that is quite puzzling."
He explained that when sperm cells begin to develop they are "locked away" in the testes so the immune system does not have access to them and destroy them.
"This study is suggesting there are other ways in which sperm can evade the immune system.
"In terms of fertility, women are exposed to these foreign bodies constantly but very few develop antibodies."