Our body is always on the lookout for PAMPs, pathogen-associated molecular patterns. These are molecules foreign to our body that are associated with infection, such as components of bacterial cell walls. We have immune cells with pattern-recognition receptors that recognize these “nonself” signatures. Not all bacteria are pathogenic, though, so, in order to be more accurate, the name was changed to MAMPs, microbe-associated molecular patterns. The beta-glucans in yeast and mushrooms are major MAMPs.
