Researchers at the Beth Israel Deaconess Medical Center say that gamma delta T cells may be able to remember harmful bacteria - then launch a powerful attack if they return.

However, their experiments on macaque monkeys may not be an accurate guide to what happens when humans get TB, says a British expert.

Scientists are keen to develop a new TB vaccine - there are continuing fears that the BCG jab, given to most British schoolchildren, is becoming less effective at preventing illness.

Trials of new versions, some using fragments of the TB bacterium itself, are underway.

However, the latest US study suggests that a vaccine which harnesses the newly-discovered qualities of gamma delta T-cells might be possible.

Memory cells

Broadly, the immune system works in two ways - the innate and the acquired systems.

The innate is the first line of defence, attacking harmful bacteria within hours of infection.

Gamma delta cells are generally considered to be part of this response.

The body then learns about the bacteria, and stores the information in the "acquired immune system", so that if infection happens for a second time with the same bacterium, the body will be able to attack and destroy it before it causes illness.

The research team looked at monkeys that had been infected but had overcome BCG, a bacterium closely related to TB.

They were then reinoculated with BCG - and the response of their gamma delta T cells measured.

Growth in numbers

Four to six days after the second BCG injection, there was a marked expansion in the gamma delta T cells in the blood of the monkeys.

This, say the researchers, means that these cells play a role in the "memory" part of the immune system.

Professor Norman Letvin, from Beth Israel Deaconess Medical Center, said: "This study, for the first time, shows that this population of gamma delta T-cells straddles the boundaries between innate and more mature immunity.

"This shows that these lymphocytes actually have the ability to develop memory. it could change how we approach the development of a vaccine to fight TB."

However, Professor Simon Carding, an expert in immunology from Leeds University, said that other studies, in both animals and humans, had failed to produce a similar effect.

He said: "I can't see this influencing the strategy we use to look for new vaccines for TB."

The paper was published in the journal Science.