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.