• Pollen - This is the cause of hayfever. Pollen is produced by plants in order for them to reproduce and the tiny grains, which are invisible to the naked eye, are spread easily by the wind. Different types of pollen will aggravate different sufferers. In the UK, grass pollen is the most common irritant, but tree pollen and weeds like nettles can also cause symptoms. Pollen levels tend to be higher in the morning and early evening, and on windy, humid days. Rain washes pollen from the air so levels drop.
• Fungal spores or mould - Moulds and yeasts are reproduced via spores. These tiny spores can lie dormant for long periods, but given the right environment - moisture - they can germinate and cause mould growth.
• House dust mites - The protein that is found in house dust mite droppings is the main cause of year-round allergies. House dust mites are microscopic organisms that live in dust in homes and workplaces and thrive in warm, humid conditions.
• Animals - Pets such as cats and dogs shed dander (mixture of fur, scales and saliva) and these can trigger an allergic reaction in sensitised people.
• Latex - This is a protein found in natural rubber, particularly in the thin, stretchy rubber that is used in surgical gloves, condoms and balloons.
• Insect venom (stings) - The venom of some insects, such as bees and wasps, contains substances that can trigger an allergic reaction. In very sensitive individuals they can trigger an anaphylactic shock.
• Food and medicine - Food allergies are caused by specific proteins in foods. Commonly shellfish, peanuts, milk, eggs, soy and wheat. This should not be confused with food intolerance. Some medicines can cause an allergic reaction too, such as penicillin.
• Chemicals - Some people find that the chemicals such as irritants in household cleaners and certain cosmetics ingredients in certain products, cause an allergic reaction.
• Metals such as nickel in jewellery or belt buckles can cause symptoms such as a rash.
  
  

Note - Allergies can take time to develop, which means the first time someone comes into contact with an allergen there may be no reaction at all, yet after repeated exposure, the body may become sensitised.