How do allergies develop?

Doctors are still not sure how people become allergic. Experts know it begins with what is called sensitisation - a period that ranges from a few weeks to several decades, in which repeated exposure to a particular allergen activates the immune system to attempt to fight what is usually a perfectly harmless substance. Because this period of sensitisation varies so much, one person can develop a contact allergy during infancy while another may not be sensitised until adulthood. Still, most allergies become apparent in childhood - especially inhalant reactions like hay fever. Some children can outgrow one allergy only to develop another later in life. Adults rarely develop new allergies after age 40 and rarely outgrow those they already have, although sometimes reactions become less severe with age.