PARIS -- US scientists have discovered that a component of the immune system that was previously deemed a blunt weapon against microbes turns out to be rather sophisticated with a memory of past invaders.

The surprise finding about so-called "natural killer" cells could unlock a new avenue in vaccine research, the authors hope.

Natural killer, or NK, cells have until now been classified as part of what is known as the innate immune system.

This is considered to be a basic response system that, in human evolutionary terms, is ancient. Its defenses react to infection each time as if it were a new event.

NKs have the role of destroying an infected cell. They are activated by cytokines -- distress signals sent out by infected cells -- and by tags on the infected cell that are attached by antibodies, the frontline forces in the immune army.

The other category of immune cells fall into the so-called adaptive immune system. These lymphocytes, called B and T cells, have "immunological memory," meaning that they recall past intruders which come back.

Vaccines work because they prime this immunological memory.

By introducing a disabled pathogen (or a fragment of it) into the body, the adaptive immune system is educated into identifying and destroying an uninvited guest years or even decades later.

In a study published online by the British-based journal Nature, microbiologists led by Lewis Lanier, a professor at the University of California, San Francisco, infected lab mice with a well-studied disease called cytomegalovirus.

They examined the NK cells from the infected mice and found that their surface was studded with receptors, or docking points, that enabled them to target virus-infected cells.

They then monitored these telltale cells over the following weeks and months and tested the animals again.

Some of the NK cells holed up in the lymph tissues after infection -- emulating the lymphocytes of the adaptive immune system -- in order to react faster and better to a returning virus.

The team transferred some of these "memory" cells to uninfected mice and then exposed them to the same virus. Within a week, the rodents had a large population of virus-specific NK cells that protected them against the disease.

The discovery calls for a "paradigm shift" in thinking about NK cells, the paper says.

Instead of being a grunt trooper from our primeval defenses, NKs appear to have characteristics of both the innate and adaptive systems.

"This changes the mindset," Lanier told AFP.

"This ability to have memory isn't something completely new that came along with B and T cells, it also exists in cells that we have previously considered innate.

"It also opens up the question whether you could actually vaccinate NK cells as well as the B and T cells to combat infections, for example, in AIDS."

Previous research has found that some individuals have a greater resistance to the human immunodeficiency virus (HIV) than others, and take far longer to progress to AIDS.

Some of this work has suggested that the difference could be partly explained by individually-tailored NK receptors that are more effective at combating HIV-infected cells than others. But the evidence so far is only statistical, and is not direct.

"If you could figure out these specific NK cells, maybe then you could boost their activity and make control of the virus much better," said Lanier.