Most of the body's immune cells self-destruct after they have attacked foreign invaders such as bacteria and viruses.

However, a special type of cell, known as memory T lymphocyte, remains on patrol after the immune system has defeated an infection.

If the same invader turns up again, the memory cells recognise it instantly and launch an early counter-attack.

Scientists believe these memory cells can be used to develop far more effective vaccines.

Until now no vaccine has been able to produce significant numbers of the memory cells. American researchers from the University of Chicago believe they have discovered why.

They say memory T cells are slow learners, and it takes several generations of intense instruction to "train" them.

This means it takes prolonged exposure to an invader to create large numbers of memory cells.

Assistant Professor of Pathology Philip Ashton-Rickardt said: "This finding suggests that the typical approach to vaccines for treatment of cancer or Aids is not often likely to produce the desired result.

"But it also shows us how we can get around the problem."

The Chicago team discovered that most T cells will die after defeating an invader.

Only a few survive and develop into memory cells, which remain eternally vigilant for any subsequent attack.

But without strong stimulation by an invader for three to four days, few memory cells emerge.

Normal vaccines administer a low dose of the foreign material which triggers an immune response, and this is insufficient to generate memory cells.

Mice experiments

Powerful new vaccines could be on the way

The scientists, whose research was reported in the journal Science, grew memory T cells in the laboratory.

This enabled them to challenge the cells constantly with high levels of foreign protein for four or five days.

They then injected the activated cells into mice without immune systems.

Ten weeks later, the injected T cells were still effective and able to react to the invading protein they had been programmed to attack.

Professor Ashton-Rickardt said: "The great thing was that the memory cells were primed and ready.

"They remembered and went after their targets as soon as they were exposed to them again.

"Although mice with normal immune systems take two or three days to mount an immune response, these memory T cells responded immediately."

The discovery could be especially useful in the treatment of the Aids virus, HIV.

People infected with HIV normally lose a different kind of "helper" T cell whose chemical signals are needed to activate killer T cells.

Although killer cells are not damaged by HIV, the loss of their helpers means they are rendered powerless.

Killer memory cells programmed to attack HIV, grown outside the body and then injected into a patient, could be an effective weapon against the virus.