system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:why do people have allergies? And what in different organisms makes people allergic?
gpt
It’s not known why some people have allergies. Allergies are an overreaction of the body’s immune system to a substance that is harmless to most people, a mistake. The substance causing the allergic reaction (the allergen) is treated as an invader by the immune system, like a virus or bacteria. The immune system produces antibodies in response, cells which release chemicals into the bloodstream to defend against the allergen. These chemicals end up causing allergic reactions, which can vary in intensity. Once the immune system has ‘tagged’ this substance as harmful, that is the pattern that remains (any future encounters will cause the same response.