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,
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