While viruses and bacteria regularly manage to infect the human organism, fungi only very rarely succeed. The reason for this is that the human immune system can recognize them very easily because their cells are surrounded by a solid cell wall of chitin and other complex sugars. Chitin is, so to speak, the alarm signal for our immune system, to which it reacts with a whole arsenal of defensive weapons. Some fungi, however, have apparently learned to avoid this fatal recognition: They possess one or more enzymes called chitin deacetylase, which they use to alter some of the chitin building blocks. This produces a chitosan, which is invisible to the immune system.
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Source: Phys.org