Our immune system has evolved to recognise molecular patterns that are commonly found in pathogens (disease-causing microbes), but not in our own cells.
These might include certain proteins, DNA/RNA structures and surface molecules unique to bacteria, viruses and other microbes. Good bacteria lack these patterns or have molecules that signal their status as harmless inhabitants.
The immune system detects these pathogen patterns through receptors and will mount an attack against anything displaying them. At the same time, it tolerates and even cooperates with microbes that lack these threatening signatures, allowing our bodies to coexist with beneficial bacteria.
This article is an answer to the question (asked by Kiera Simons, via email) 'How does our immune system distinguish between good and bad bacteria?'
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