When you have an allergic reaction what is released that makes you itch and causes your eyes to swell up?

During an allergic reaction, the immune system releases histamines, which are chemicals that play a key role in the body's response to allergens. Histamines can cause symptoms such as itching, swelling, and redness by increasing blood flow and causing inflammation. This is why you might experience itching on your skin and swelling around your eyes when exposed to an allergen.