Histamine intolerance is quite simply an imbalance of histamine and the enzyme DAO in the body. This deficiency means that your body is not in the position to break down ingested histamine quickly enough.
The consequence
Ingested histamine is broken down significantly more slowly and accumulates within the body. The body then reacts to the accumulated histamine with similar symptoms as for allergies. It is important to know however that this does not point to an allergy, but to an intolerance.