Why some people get sicker than others when exposed to same illness

WASHINGTON - A new study conducted on a population of wild sheep located on a remote island off the coast of Scotland that gauged the animals’ susceptibility to infection may shed new light on why some people get sicker than others when exposed to the same illness.

The answer to this medical puzzle may lie in deep-rooted differences in how animals survive and reproduce in the wild, according to the study, which was led by Princeton ecologist Andrea Graham.

The research from Princeton University and the University of Edinburgh scientists revealed that the sheep population over time has maintained a balance of those with weaker and stronger levels of immunity and fertility.

“This is a groundbreaking study that to my mind will change our whole understanding of the immunoheterogenity in animal populations,” said Peter Hudson, the Willaman Professor of Biology and director of life sciences at Penn State University.

“Graham and colleagues show beautifully the tradeoffs in the immune system as a balance … that maximizes reproductive output.”

Graham, an assistant professor of ecology and evolutionary biology at Princeton who also is on the faculty of the University of Edinburgh, led the study of wild Soay sheep on the remote island of Hirta in the St. Kilda archipelago, about 100 miles west of the Scottish mainland.

The scientists tested the animals for levels of antibodies, natural molecules produced by the sheep’s immune systems to fend off infections such as influenza or those caused by parasitic worms.

The sheep whose blood contained the most antibodies lived the longest, the researchers found. These animals also were most likely to survive harsh winters. However, they failed to produce as many offspring each spring as other sheep. Sheep with lower levels of antibodies tended to die earlier, they found, but also gave birth to more lambs each year.

Viewed in terms of breeding and, ultimately, evolutionary success, the differing groups of sheep were equally successful in that the longer-lived but less fertile sheep and the shorter-lived but more fertile sheep produced about the same number of progeny over the course of their lives. The overall balance, the researchers said, could help explain why immunity varies so much among individuals.

The study has been published in the Oct. 29 issue of Science. (ANI)