According to a new study by scientists in the United Kingdom and the United States, a strong immune system can increase the lifespan of an animal but it leads to lower fertility. The findings, published in the journal Science, they explain why some individuals are more susceptible to infection than others.
The strength of the immune system varies greatly from person to person. The question is why evolution has not eliminated the genes associated with a weaker immune system. One possible reason is that a strong immune system can lead to lower fertility.
“We suspected for some time that strong immune responses to prolong the life against infection, but at the same time affecting the playback,” said Andrew Graham, University of Edinburgh in the UK and Princeton University in the United States. “Finding evidence of this compromise could clarify why there are such large variations in the strength of immune responses in animals and their susceptibility to infection and autoimmunity.”
To see clearly, Professor Graham and his colleagues studied wild Soay sheep on a remote uninhabited island in the archipelago of St Kilda Hirta which is located about 160 kilometers (km) off the west coast of Scotland.
Every August for 11 years the researchers collected blood samples of animals and have noted how many lambs each ewe had had and how long the animals lived.
The team was concerned at the levels of antibodies in the blood of animals, the sheep with high levels of antibodies have stronger immune systems and are better equipped to combat parasitic infections, which are commonplace among the sheep of Hirta.
The analysis revealed that sheep with high antibody levels lived longer and were more likely to survive harsh winters. They had, however, less likely to have offspring than other animals.
In contrast, sheep with relatively low levels of antibodies give birth to more lambs every year but had a shorter life than the sheep with high antibody levels.
In fact, the evolutionary point of view, both groups had the same yield and produced about the same number of children throughout their lives. According to the researchers, this may help to explain why immunity so different from one individual to another. Moreover, the trend towards stronger immune systems or weak seems to be hereditary. “This means that the genetic basis of natural selection has the ability to model the stretch,” said Professor Graham.
Furthermore, the results suggest that autoimmunity, in which the immune system is so strong that it attacks the tissues of the body, can exist in nature. Until now, the autoimmunity was seen only in humans and in laboratory mammals, domestic and captive. It is necessary to continue the search for further investigate this issue, the scientists explain.
Lynn Martin University of South Florida in the United States, which has not participated in the study, said that the use of wild animals by researchers is significant.
“For a long time the field of immunology has been based on the study of pets in laboratories where animals were given clean all the food they wanted, where they were protected from the weather and had almost no problems with pests,” he said. “These are good conditions to identify all the details on how to handle guests parasites at the molecular and cellular level, but do not represent the natural conditions and may affect our understanding of immune function.”
The results of this study instead help to explain why some suffer more than others when exposed to infection and because the vaccines are more effective in some people than others.
