Researchers are undertaking a study to locate genes that help people live long lives, an effort they hope will help unlock some of the mysteries of aging.

Siblings 85 and older are being sought by researchers who plan to examine possible links between the immune system and longevity, said Dr. Jaime Durand of Hines Veterans Administration Hospital.Although humans have an estimated 100,000 genes, Durand says animal studies have provided clues about where to start looking.

"The longevity genes have been shown in one study in mice to be associated with genes controlling the immune response," Durand said. "Mice that produced the strongest antibodies to viruses and bacteria lived longer."

Dr. Jacob A. Brody, former associate director of the National Institute on Aging and now dean of the University of Illinois School of Public Health, said the study may provide valuable insight into aging.

"Thus far, animal work has been tantalizing, but it has relativley limited use in human aging," he said.