system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can some people be allergic to cats but not dogs?
gpt
All cells have surface proteins. When you inhale, get a cut, eat something, etc... and your body internalizes foreign cells, it creates an immune response. If the immune system says "this could hurt us!" it will fight it, even if its harmless, like cat or dog dander. This immune reaction to harmless things is an allergy. Cat and dog cells have different proteins recognized by your body, so if you're allergic to one and not the other, your immune system at some point in your life overreacted and created a response, thinking it was something infectious.