We have the all-important immune system to thank for protecting our bodies from all kinds of pathogens — everything from bacteria and viruses to cancer cells and toxins to parasitic worms. The immune system is a hard-working, tightly orchestrated collection of organs, substances, and cells that produces pathogen-fighting antibodies to keep us healthy. As we age, our immune systems weaken, making us susceptible to life-threatening illness and infection (influenza and pneumonia, for example, which together are the eighth leading cause of death in the U.S. and the fifth leading cause for those over 65). Now, scientists have pinpointed a reason why our immune systems eventually flake out on us.

White blood cells are the foot soldiers on the front lines of the body’s fight against invading pathogens, but red blood cells are no slackers either, carrying oxygen to the body’s tissues via the blood. A new study by a UC San Francisco research team, published Wednesday in the journal Nature, has revealed the process by which blood cell production slows as we age. Hematopoietic stem cells, which maintain their own numbers through cell division, produce both white and red blood cells — replacing them when they die, and keeping their levels stable. It’s these stem cells that could hold the key to longer life.

The UCSF researchers found that as the stem cells grow older, they replicate their own DNA less and less accurately, meaning that the copies they create of themselves are less and less efficient at producing blood cells. The ones they do produce are at greater risk of DNA damage and death. Aging stem cells especially lose the ability to produce B-cells, which fight microbial infections such as pneumonia. The researchers noticed that the old, decrepit stem cells they studied had something in common: scarcity of a certain protein component that they need to replicate their DNA faithfully and create new, healthy copies of themselves.

It’s the discovery of this protein deficit that has scientists so excited. They theorize that a drug could be developed to restore these protein components to the stem cells, head off immune-system failure, and significantly extend the human life span. “Everybody talks about healthier aging,” explained Emmanuelle Passegué, PhD, a professor of medicine at UCSF and the lead researcher in the study. “The decline of stem-cell function is a big part of age-related problems. Achieving longer lives relies in part on achieving a better understanding of why stem cells are not able to maintain optimal functioning.” Soon, living to 121 years old may not be such an anomaly.