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 tendency to form either strong or weak responses to infection ran in families in the sheep, the researchers also found. "This genetic basis means that natural selection has the chance to shape the trait," Graham said. If differing responses to infection still result in equal long-term reproductive success, she said, this means "selection seems actually to be maintaining this genetic variation in immunity."
Such a balance could help explain why vaccines seem to protect some people better than others or why some people get sicker than others when exposed to the same infection.
"We have long suspected that strong immune responses should prolong life in the face of infections, but might also be costly to reproduction," Graham said. "To find evidence for such tradeoff may clarify why animals vary so much in the strength of their immune responses, and even in their predisposition to infection or autoimmunity.
The technique used to study immunology in the wild was vital to the importance of the study, according to Lynn Martin, an assistant professor in the department of integrative biology at the University of South Florida.
"For so long, the field of immunology has been based on studies of domesticated animals in clean lab environments where animals are given all the food they want, shelter from the elements and few to no challenges with parasites," Martin said. "These conditions are great to get at the details of how hosts are dealing with parasites at the molecular and cellular
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