Why does your skin swell when you get stung by a wasp?

Wasp sting. A wasp sting swells because the sting is accompanied by release of a venom which produces inflammation at the site of the sting. This causes the swelling.
Wasp Stings. Wasp stings release venom which may have as many as 13 antigens. Enzymes in wasp venom break down cell walls ; degranulate mast cells which in turn leads to release of histamin resulting in symptoms like itching, swelling ; pain.