Trained Immunity What’s that?! THE INNATE AND ADAPTIVE IMMUNE SYSTEM Our immune system is broadly divided into two separate “arms” that communicate with each other to protect us from infections. These two factions are termed the innate and adaptive immune systems. The innate immune system is evolutionarily more ancient than its adaptive counterpart. It acts as a broad first-line of defense when we get infected with pathogens (think viruses, bacteria, and fungi). For example, a particular innate immune cell, termed a macrophage, can “eat” whole bacteria when it encounters them. If the innate immune system is unable to clear the infection, it coordinates with the adaptive immune system to recruit reinforcements. Adaptive immunity (T and B cells) is more specialized than its innate counterpart, and therefore can mount highly specific responses, such as antibody production, against particular structures present on a given pathogen. Due to their highly specific nature, adaptive immune responses are often sufficient to clear an infection. Even after an infection is cleared, a small portion of pathogen-specific B and T cells remain in our bodies as memory cells to protect us from another infection.
munity and has received a lot of interest by scientists over the past few years. Importantly, in certain situations, innate immune memory may be even “better” than memory by adaptive immune cells, due to its ability to protect us from a wider array of infections. This is especially true for the BCG vaccine, whose ability to reduce infant mortality worldwide cannot be explained solely by its tuberculosis-specific protective effects.
MITOCHONDRIA TCA CYCLE
METABOLITES
WHAT IS IMMUNE MEMORY? Now what happens when we encounter the same infection again? Say we were vaccinated against tuberculosis with the BCG (or “Bacille Calmette-Guerin”) vaccine, then how would our immune system respond when we potentially encounter the causative bacteria of tuberculosis? Scientists previously thought that only the memory B and T cells will “remember” the 1st infection and will protect us when we get infected again. This is the principle behind how vaccines work; we have been mildly “infected” during a tuberculosis vaccination, and when we encounter the Mycobacterium tuberculosis bacterium later in our life, the memory B and T cells respond against it quickly, thereby protecting us from getting sick. However, over the past years, scientists have learned that innate immune cells (remember the 1st line defenders?) can also remember previous infections. If you are surprised at this, you are not alone – scientists were too! This new discovery of memory by innate immune cells has been coined trained im20 IMMpress Vol. 11 No. 2 2023
DNA MODIFICATIONS
EPIGENETIC REPROGRAMMING
