system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What are hives? (The skin condition)

What in the body causes them, what are they made of, and how do they help keep the body safe?
gpt
Hives (the technical term is Urticaria) have a number of causes but are mostly caused when something that your body doesn't like meats a special white blood cell called mast cell. This thing could be a protein from a peanut or a bit of medicine which your body should be ok with but for some reason isnt. We call this an 'allergen'.

When the allergen meets the mast cell it 'activates' it and the cell bursts causing a substance called histamine to pour into your blood stream.

Histamine is large quantities is very bad for your body. It causes the walls of the blood vessels to open up and allows fluid to move from the blood into the space around your cells.

In the skin all this leaky fluid can cause swelling, itching and redness (called inflammation) - which is what yoy see as hives. If there is enough histamine this can also cause swelling around the throat and vocal cords so the person cant't breathe and a lot of leaking can leave too little blood where the blood is supposed to be. This causes life threateningly low blood pressure.

Thats allergic hives but some people can have a hives when only the skin reacts and releases histamine. This can be caused by plants - poisen ivy is a good example - or insect bites and stings or by normal things like sweating, hot or cold temperatures and stress but essentially it's all the body interpreting something harmless and something harmfull.  So essentially hives are your body overreacting to something.

In terms of how they keep your body safe, they do make you remove yourself from something your body thinks is a harmful - eg plants, insect bites, but in the modern day they are mostly an unhelpful nuisence.