Wednesday, September 23, 2009

Evolution and Immunosenescence

As we age our immune system declines. Why is this so? This paper in the June issue of Trends in Immunology tackles that question. Here is the abstract:

There is an accumulating body of evidence that a decline in immune function with age is common to most if not all vertebrates. For instance, age-associated thymic involution seems to occur in all species that possess a thymus, indicating that this process is evolutionary ancient and conserved. The precise mechanisms regulating immunosenescence remain to be resolved, but much of what we do know is consistent with modern evolutionary theory. In this review, we assess our current knowledge from an evolutionary perspective on the occurrence of immunosenescence, we show that life history trade-offs play a key role and we highlight the possible advantages of the age-related decline in thymic function.

And a sample:

Potential pathogens—viral, bacterial, fungal, macroparasite and dysfunctional host cells—present a major threat to survival, and the innate and adaptive immune systems have evolved a series of defence networks to protect the individual from such harmful agents. These systems are not without fault, however, and with increasing age, problems arise in functional activity. There is clear evidence of an age-related decline in effectiveness of the immune systems of vertebrates and some invertebrates, which renders older individuals more vulnerable to infection.

....The adaptive immune system with high specificity to antigens first appeared some 350 million years ago in jawed fish, and its value is demonstrated by the fact that it has been retained, albeit with a variety of modifications, by all vertebrates.

....The force of natural selection declines with age, leading to a particular value being placed on survival to maturity and on reproductive output thereafter. An ability to mount a strong inflammatory response early in life and the finely tuned repression of the immune response to accommodate successful reproduction are clearly in support of this.