Researchers are closer to understanding what animals need to regrow their body parts, after Australian scientists established the key role of the immune system in salamanders.

Also known as Mexican walking fish, salamanders can regenerate their own limbs, tails, jaws, retina and heart. But the researchers found that if certain immune cells were blocked, the amphibians were unable to regrow limbs, though healing did occur.

"It means that we have turned this perfect process of regenerating a limb into a failure of the kind you would normally see in mammals," said lead researcher James Godwin, of Monash University's Australian Regenerative Medicine Institute.

Published in the Pro-ceedings of the National Academy of Sciences today, the research explains the previously unknown role of immune cells, known as macrophages, in the amphibian's ability to regrow arms and legs.

The results have implications for humans, who also have these immune cells – it could lead to ways to tweak the human immune system, putting it on a more regenerative path for both limbs and other body parts.

In salamanders, the new tissue is scar-free. This has benefits for liver and heart disease, which are linked to fibrosis or scarring.

"Now we have a really good idea of what is required for perfect regeneration," Godwin said of his work with colleagues Nadia Rosenthal and Alexander Pinto.

"We have a smoking gun. If we can find out what they deliver to make regeneration occur, then we might be able to tweak the human wound-healing scenario."

Godwin calls macrophages "the guardian angels of the body".

The highly mobile immune cells can communicate with other cells and gobble up debris or harmful bacteria. They arrive at the wound site within six days of amputation.

"If the macrophages are not present in the early phases of healing, regeneration does not occur," he said.

The researchers used "poisoned lipids" to block the immune cells in salamanders. With the immune cells disabled, the animal lost the ability to regrow limbs and instead the wound healed as an amputation would in humans.

"You get a limb stump with lots of scarring at the tip," Godwin said.

"It shows that those immune cells are required to be there to oversee and manage the early stages of wound resolution. If they're not there then the whole thing goes wrong."

The goal is to make a drug that when applied to a wound would make the human response more like a salamander's.

"Humans, like salamanders, grow limbs – we can do it," Godwin said.

"It's just that it's been turned off and we have to work out if we can turn it on so we can regenerate our own limbs."