The innate immune system evolved long before the adaptive immune system arose as a more sophisticated layer atop it. It is generally considered that only jawed vertebrates have an adaptive immune system, but there are interesting examples of stranger, adaptive-like innate immune systems in some of the more ancient jawless vertebrate lineages, such as lampreys. An overly simplistic view of the difference between innate and adaptive immunity is that the innate immune response is always the same, that for a given stimulus it will respond in the same way tomorrow as it does today. The adaptive immune system, on the other hand, maintains a memory. It will respond far more quickly and efficiently to any future incidence of a stimulus that it has encountered in the past.

Nothing in biology is simple, however. Researchers have become aware that the innate immune response in mammals can in fact change over time in response to stimuli, a phenomenon termed trained immunity. This appears to be an epigenetic process, and thus may or may not be truly lasting for any given individual - it may fade over time, if the stimulus is removed. Nonetheless, in the open access commentary noted below, researchers suggest that trained immunity may contribute to the age-related decline of the immune system into chronic inflammation and incapacity.

The impact of persistent infection or overall lifetime burden of infection on immune aging is more usually considered in terms of its effects on the adaptive immune system. Since the supply of new T cells declines with age as the thymus atrophies, the adaptive immune system behaves ever more like a resource-limited system. Only so many T cells that can become devoted to memory or to specific threats before there are too few naive T cells remaining to effectively tackle new pathogens. Prevalent and persistent herpesviruses such as cytomegalovirus are considered to be the most important burden in this sense, and the immune system uselessly devotes ever more resources to futile attempts to remove these viruses. It is interesting to consider that an analogous harmful reaction to persistent infection may be taking place in the innate immune system as well.

Be aware, innate immune cells remember