The highest levels of DAO expression are observed in the digestive tract and the placenta. In humans, a certain subtype of cells of the placenta, namely the extravillous trophoblasts, express the enzyme and secrete it into the blood stream of a pregnant woman. Lowered diamine oxidase values in maternal blood in early pregnancy might be an indication for trophoblast-related pregnancy disorders like early-onset preeclampsia. Normally the enzyme is not or only very scarcely present in the blood circulation of humans, but it increases vastly in pregnant women suggesting a protective mechanism against adverse histamine. It is also secreted by eosinophils. In case of a shortage of diamine oxidase in the human body, it may appear as an allergy or histamine intolerance.