PSA is a bacterial molecule: exactly the type of substance that, according to common wisdom, the immune system should see as a threat. PSA ought to trigger inflammation. Instead, it does the opposite: it quells inflammation and calms the immune system. Mazmanian calls it a ‘symbiosis factor’ – a chemical message from microbe to host that says: I come in peace.27 This clearly shows that the immune system isn’t innately hard-wired to tell the difference between a harmless symbiont and a threatening pathogen. In this case, it’s the microbe that makes that distinction clear.
