As part of the inflammatory response, cells release histamine when they are injured. When cells release excessive amounts of histamine, an antihistamine (a pharmaceutical drug that inhibits the action of histamine) is often required.
It would appear that an over-production of histamine in the body is an over-reaction by the cells to an enemy or threat. This over-production of histamine often results in allergies that can need a pharmaceutical to settle them down.
Along the same lines, could the need to take an antihistamine be an indication that a person is over-reacting to someone who is irritating them, or an intolerance to something that another person is doing?
On the physical level, the threat to the cells is mostly related to physical trauma, intense heat, irritating chemicals as well as infections from viruses and bacteria. On a metaphysical level, could the threat to the person also be related to a physical action, intense anger, intolerance, irritation by someone else, or people who are invading their space?
As always, the important clue is: when did the symptom begin and what was happening at the time.
With any allergies affecting the nose, eyes, throat or sinuses, the reflexes in the toes or fingers are a good place to begin.