What happens when u brake out in hives?

Usually . It is caused by mediators such as histamine released by mast cells and basophils in your tissue.
You itch a lot. Mast cells release histamine. Histamine combines with receptors on capillary lining cells & sensory nerves adjacent to the mast cells. Nerve cells fire giving the sensation of itching. A nervous reflex stimulates the blood vessels to dilate increasing blood flow producing redness. Spaces open up between the capillary lining cells allowing plasma to leak out causing swelling.