system
Given the following post, respond as if you're commenting on Reddit:
human
If the immune system can mistake a harmless substance as an invader (ie. allergies), can it mistake a HARMFUL virus/bacteria as harmless? If not why?
gpt
You seem to misunderstand the way the immune system is 'trained'. Adaptive immune cells are not given a catalogue of antigens to attack. If that were the case, how could they possibly target newly mutated pathogens?

Instead, they are taught what *not* to attack. Unfortunately, this system can backfire and produce cells capable of launching an adaptive response against things they shouldn't. For example, a bit simplified, the cause of allergy is the adaptive response against a harmless protein, which can lead to over-inflammation. Alternatively, an adaptive response can be launched against a self antigen - the cause of autoimmunity.

So, to answer your question, pathogens can indeed avoid the immune system, but not in the way that, for example, cancer cells do it. It really can't result that way because of the training process of immune cells.