New York, Dec 12 (IANS): After examining DNA from 21 primate species - from squirrel monkeys to humans - scientists have exposed an evolutionary war against infectious bacteria over iron that circulates in the host's bloodstream.

Following infection, the familiar sneezing, running nose and inflammation are all part of the immune system's attempts to shield the body from hostile invaders.

Lesser known is a separate defence against invasive microbes, called "nutritional immunity" that quietly takes place under our skin.

This defence mechanism starves infectious bacteria by hiding circulating iron, an essential nutrient it needs for survival.

The protein which transports iron in the blood, transferrin, tucks the trace metal safely out of reach.

"Over the last 40 million years of primate evolution, this battle for iron between bacteria and primates has been a determining factor in our survival as a species," said Matthew Barber, postdoctoral fellow in human genetics at the University of Utah.

Clever as it sounds, the ploy is not enough to keep invaders at bay.

Several bacterial pathogens - including those that cause meningitis, gonorrhea, and sepsis - have developed a weapon, transferrin binding protein (TbpA), that latches onto transferrin and steal its iron.

Though scientists have known of the offensive strategy, they failed to realise how pivotal the battle over iron has been in the conflict between host and pathogen.

"Understanding the strategies that underlie natural defence mechanisms, including 'nutritional immunity', could inform new approaches to combating antibiotic-resistant bacteria and emerging diseases," Elde concluded.

The paper appeared in the journal Science.