system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does a cold causes blood vessels in the nasal passages to swell and, as a result, a stuffy nose?

I'm aware a stuffy nose is caused not by mucus, but by "swollen blood vessels in the nasal passages". But why does such swelling happen? How exactly does a cold causes this?
gpt
When your body detects that a foreign substance has entered the nose (including things like viruses, pet dander, pollen, and other airborne irritants) your immune system releases histamines. Histamines bind to alpha and beta receptors in the mucosa at a molecular level and tell the body to increase blood flow to the sinuses and turbinates. The blood vessels and capillaries become larger and you get a stuffy nose. Antihistamines block this action by antagonistically binding to these same receptors which prevents histamine from binding and causing a reaction. Antihistamines will reduce the itch you feel, while nasal decongestants will actively shrink the blood vessels and capillaries so you can breathe again.