Allergies can be caused by two things: host and environmental factors. Host is if you inherit an allergy or are likely to get it due to your age, sex or racial group. Environmental factors can include things such as pollution, epidemic diseases and diet. People who are likely to develop allergies have a condition known as ‘atopy’. Atopy is not an illness but an inherited feature, which makes individuals more likely to develop an allergic disorder. Atopy tends to run in families.
The reason why atopic people have a tendency to develop allergic disorders is because they have the ability to produce the allergy antibody called ‘Immunoglobulin E’ or ‘IgE’ when they come into contact with a particular substance. However, not everyone who has inherited the tendency to be atopic will necessarily go on to develop an allergic disorder.
Barnaby Salton, Science Museum