SYDNEY, Aug. 15 (Xinhua) -- Australian scientists have made a breakthrough in solving the mystery of why an organ responsible for human immune defense shrinks and weakens with age, according to a study published on Thursday.

In a world-first, researchers from the Walter and Eliza Hall Institute of Medical Research (WEHI) in Melbourne discovered new cells that drive the aging process in the thymus - a finding they say could unlock ways to prevent immunity waning with age.

Located in the upper chest under the breastbone, the thymus is the only organ in the body that can make T lymphocytes - also known as T cells - a type of white blood cell that helps fight against pathogens and eliminate infected or cancerous cells.

However, the thymus is also the first organ in the body to start shrinking with age, causing diminished T cell production and a weakened immune system.

The WEHI team discovered two new cell types that cause the thymus to lose its function.

By using advanced imaging techniques and animal models, researchers found that the cells, which were found only in the defective thymus of older humans and mice, form clusters around T cell growth areas, impairing production, and "scars" in the thymus that prevent the organ from restoring itself after damage.

WEHI Laboratory Head Professor Daniel Gray said T cell production declines significantly after puberty and virtually ceases by age 65, making it harder for the body to deal with new infections and cancers.

"This is also why adults who have depleted immune systems, for example due to cancer treatment or stem cell transplants, take much longer than children to recover," he said in a media release.

The discovery provides a new angle for thymic regeneration and immune restoration, and could unravel a way to boost immune function in vulnerable patients in the future," Gray said.

It is the first time that research has proven that changes that occur inside the thymus impact its ability to function with age.

The study was conducted in collaboration with researchers from the Fred Hutch Cancer Center in Seattle and Memorial Sloan Kettering Cancer Centre in New York. ■