First, don’t run around screaming. That’s for movies. Wash the bite with soap and water. It’s basically first aid for any life oopsie.
Apply a cold pack. Or a bag of frozen peas. Peas work great because they mold to your arm. Plus, you can eat them later if you’re brave.
Elevate the bite if it’s on a limb. Unless you’re a contortionist. Then just tie it in a knot — whatever feels right.
Pop an antihistamine if it itches. Scratching is fun, but it leads to scabs. Scabs are spider battle scars no one asked for.