Wikipedia’s featured article today is about the immune system. This article is well worth reading because understanding the immune system is central to understanding how allergies operate at a biological level.
The article points out that even single-celled organisms like bacteria have a defence system against viruses. As life evolves and becomes more complex, the immune system too must become more complex.
Its a cat-and-mouse game between the immune system and pathogens, as bacteria, viruses and the like evolve to get past the immune system, and the immune system responds by learning how to recognize the newly disguised pathogens.
It is easy to see how such a delicately balanced system can become disrupted.
Even if you don’t understand everything in the article, it is well worth a read. There are some really great scanning electron microscope images of cells, if you like that sort of thing (I do).