The traditional understanding of the immune system was that innate immune cells couldn’t learn; that they dealt with each invader the same way every time. But new evidence suggests innate responses are changed by previous infections or vaccination, through “innate learning” or “trained immunity”.
Because innate learning changes innate immune cells, it has broad effects on how the immune system deals with infections. That means infection by one invader can cause changes in how the immune system deals with a completely different invader. In contrast, adaptive learning leads to very specific protection against repeat infection by the same invader.
Innate learning can have unexpected effects on how our immune system deals with secondary infections (infections that occur during or after a different infection). Here’s an example: the “bacille Calmette-Guerin” (BCG) vaccine is designed to protect against tuberculosis, and does a good job at protecting from tuberculosis as expected. But it also protects against infection by a completely unrelated invader, Candida albicans.
Candida albicans infection causes “candidiasis“, more commonly known as thrush or yeast infection. Studies of BCG-vaccinated patients had previously suggested that vaccination protects against infections other than tuberculosis. But how this happened wasn’t understood.
Then researchers showed protection from yeast infection happened through an improved innate response. The innate immune cells “learn” from the vaccine and protect from yeast infection for up to three months afterwards. And this is just one of a growing number of studies showing that innate immune cells can learn.
These studies extend to primitive invertebrates, including flies and mosquitoes. Invertebrates completely lack adaptive immune cells, so any immune memory responses they have are from innate learning.
In mice, herpesvirus infections can protect from completely different bacterial infections. Rather than responding the same way to every invader, innate learning changes our immune responses based on previous experiences.
Recent evidence also suggests immune learning is strongly affected by environmental factors, including diet, lifestyle, our surroundings and previous infections.
Immune responses to the yearly flu vaccine, for example, are impacted more by environmental factors than genetic differences. This suggests we can improve our immune responses by altering life experiences.
Rather that being stuck with the immune system we genetically inherit, research is showing that immune responses are shaped by life experiences. It provides hope that we can improve immunity and reduce disease through changes in lifestyle and our environment.