Meaning out of place, this is a form of hypersensitivity characterised – amongst other features – by a familial tendency. It is due to the propensity of the affected individual to produce large amounts of reagin ANTIBODIES, which stick to MAST CELLS in the mucosa, so that when the ANTIGEN is inhaled, HISTAMINE is released from the mast cell. Atopy may be responsible for ASTHMA and HAY FEVER (see also ALLERGY). It is estimated that 10 per cent of the human race are subject to atopy. (See also DERMATITIS.)