There have been a few studies on the cause of hives, something I suffered from several times in my life, and have found it is due to a histamine imbalance. Swelling, redness, itchiness, and vasodilation (widening of blood vessels), are all symptoms of this imbalance.
Histamine is a neurotransmitter that has an important role in the immune response.
It is concentrated in mast cells and when released, causes itching, increases the permeability of blood vessels and produces swelling and skin reddening.
Histamine also plays a regulatory role in muscle contraction and gastric acid secretion.
Histamine increases vasodilation causing symptoms such as hypo tension (decreased blood pressure) and tachycardia (increased pulse rate), and causes constriction of the bronchi in the lungs.
Histamine is made from histidine (an amino acid or protein fragment) and its intolerance is due to an excess of histamine which results in a variety of symptoms.
The key to lowering histamine in your body, is through diet. There are many foods that increase your body's histamine levels, and I highly recommend you research histamine intolerance and low-histamine diets, so that you can lower the level to a more tolerable amount, and your symptoms will quickly ease. I take Vitamin C supplements to keep my histamine levels lower because it serves as a natural anti-histamine. Knowing what foods to avoid (or consume in small amounts) has ended my suffering with hives. There really is something to the old saying, "you are what you eat." Good luck and I hope this information helps.