As doctors have gained control over most infectious diseases and identified effective treatments for chronic conditions, some medical researchers have begun to focus on aging itself.

We’re living longer, but we still inevitably die, and these researchers are hoping to explain why.

A study published today in the journal Proceedings of the National Academy of Science suggests that a growth factor may help extend the function of the thymus, which produces T cells — the rock stars of our immune system.

As we age, our immune system becomes weaker and less able to fight back against cancer and infections that can kill us.

The study suggests that dwindling levels of the growth factor FGF21 in the thymus make it harder for the organ to produce T cells.

The research was spearheaded by Vishwa Deep Dixit, a professor of comparative medicine and immunobiology at Yale School of Medicine. The scientists genetically altered mice to produce more FGF21 and then compared the results to standard mice.

The engineered mice had healthier thymus organs in middle age than their unmodified counterparts. With more FGF21 in the thymus, the mice had less fatty buildup. The organs also produced a wider variety of T cells.

The growth factor mice lived 40 percent longer than the control mice.

Read More: PTSD Linked to Faster Aging, Earlier Death »

Possible Treatment for Cancer Patients

The new study builds on controversial research from several years ago that found that calorie restriction, if it doesn’t result in malnutrition, can extend life expectancy.

The researchers zeroed in on FGF21 because the body produces more of it when caloric intake is restricted. The hormone allows the body to burn fat when its fuel of choice — glucose — is scarce.

FGF21 may also be effective as an injected medicine. And because it induces weight loss and boosts insulin sensitivity, FGF21 is also being considered as a possible treatment for obesity and type 2 diabetes.

If later research in human trials leads to similar findings, cancer patients recovering from chemotherapy would likely be first in line to try the treatment to see if it helps rebuild their immune systems.

“Elevating the levels of FGF21 in the elderly or in cancer patients who undergo bone marrow transplantation may be an additional strategy to increase T cell production, and thus bolster immune function,” Dixit said in a statement.

The study was funded by The National Institutes of Health, The Robert Welch Foundation, and the Howard Hughes Medical Institute.

Read More: Study Breaks Down Aging Process »