Histamine, again, is the main culprit when it comes to allergic skin reactions. Most often, these reactions come in the form of hives. These itchy, red dots or blotches result from fluid leaking out of the dilated blood vessels into the surrounding tissue. When this happens at a deeper level, it's called angioedema. Although stress can seem to cause hives, the link between these two may be that stress throws off the immune system, worsening the unnecessary reaction that's already underway.