Sheep shed light on immunity

Having long wanted to visit St Kilda, I was interested to read in Science about a study of wild Soay sheep on the island of Hirta in the St Kilda archipelago. The research could explain differences in both immunity and fertility, not only in animals, but also in humans, and why some people become ill when exposed to an infection and others do not.

The researchers evaluated antibody levels in the sheep and found those with the highest levels lived longest and were more likely to survive harsh winters, although they failed to produce as many lambs each spring as sheep with lower levels of antibodies. However, the more prolific sheep tended to die earlier.

In terms of breeding and evolutionary success, the two groups of sheep were equally successful in that over the course of their lives the longer-lived but less fertile sheep produced about the same number of lambs as the shorter-lived but more fertile sheep.

The tendency to form either strong or weak responses to infection seemed to have a genetic basis in that it ran in families in the sheep, leading the researchers to conclude that natural selection may maintain the differences in immunity. Given that variations in immunity did not influence life time fertility in the sheep in this study, there is no evolutionary harm done. This trade-off may explain why animals vary so much in immune response.

If applicable to humans, these findings could help explain why vaccines seem to protect some people better than others and why some people have a stronger predisposition to infection than others.