What are allergies?
Having an allergy means having a bad reaction to something in the air or something you eat or come into contact with.
These triggers are called allergens, and include pollen, mould, and dust in the air, as well as foods such as nuts.
Allergies are common, according to Allergy UK, one in four people in the UK suffers from an allergy at some point in their lives.
No one knows why some people develop allergies, but they may be inherited. Allergies may flare up and subside throughout your life.
What happens during an allergic reaction?
When a person is exposed to an allergen, a series of events takes place:
- The body starts to produce a specific type of antibody, called IgE, to bind the allergen.
- The antibodies attach to a form of blood cell called a mast cell. Mast cells can be found in the airways, in the gastrointestinal tract, and elsewhere. The presence of mast cells in the airways and gastrointestinal tract makes these areas more susceptible to allergen exposure.
- The allergens bind to the IgE, which is attached to the mast cell. This triggers a reaction that allows the mast cells to release a variety of chemicals including histamine, which causes most of the symptoms of an allergy, including itchiness or runny nose.
If the allergen is in the air, the allergic reaction will most likely occur in the eyes, nose, and lungs. If the allergen is swallowed, the allergic reaction often occurs in the mouth, stomach, and intestines. Sometimes enough chemicals are released from the mast cells to cause a reaction throughout the body, such as urticaria - itchy red welts (hives) - decreased blood pressure, shock, or loss of consciousness.
What are the symptoms of allergies?
Allergy symptoms can be categorised as mild, moderate, or severe (anaphylactic).
- Mild reactions include those symptoms that affect a specific area of the body such as a rash, itchy, watery eyes, and some nasal congestion. Mild reactions do not spread to other parts of the body.
- Moderate reactions include symptoms that spread to other parts of the body. These may include itchiness or difficulty breathing.
- A severe reaction, called anaphylaxis, is a rare, life-threatening emergency in which the response to the allergen is intense and affects the whole body. It may begin with the sudden onset of itching of the eyes or face and progress within minutes to more serious symptoms, including abdominal pain, cramps, vomiting, and diarrhoea, as well as varying degrees of swellings that can make breathing and swallowing difficult. Mental confusion or dizziness may also be symptoms, since anaphylaxis causes a quick drop in blood pressure.
Types of allergies
Allergies come in many distinct forms and are typically grouped in general categories according to the types of substances that cause them or the parts of the body they affect:
Skin allergies: Eczema and contact dermatitis are often caused by direct skin exposure to a specific allergen. Hives, or urticaria, is an eruption of itchy, swollen, reddened welts that can last for minutes or days. Angioedema is characterised by a deeper swelling around the eyes and lips, and sometimes of the hands and feet as well. Both hives and angioedema stem from the body's adverse reaction to certain foods, pollen, animal skin, drugs, insect stings, cold, heat, light, or even emotional stress.
Respiratory allergies: According to the NHS, up to 1 in every 5 people in the UK experience hayfever (allergic rhinitis), which frequently also causes bouts of sinusitis. Typical symptoms include itchy eyes, nose, roof of mouth, or throat, along with nasal congestion, coughing, and sneezing. If you - or members of your family - have other allergies such as eczema or asthma, you are more likely to have hayfever. Hayfever can be caused by a number of difference substances - pollens of certain weeds, grasses, and other plants whose pollen is spread by the wind, and moulds. Mould allergies are caused by spores in the air. Outdoor moulds thrive in warm seasons or climates, while indoor moulds grow throughout the year in damp locations - basements and bathrooms, for example. Dust causes allergies because it harbours offenders such as pollen, mould spores, and microscopic house dust mites - it may also contain irritating fibres from fabrics, upholstery, and carpets.
Asthma: Asthma has various causes and factors that trigger it, including respiratory viruses, environmental exposures, and allergies to pollen, mould spores, animal dander, and house dust mites.
Food allergies: Food allergies are rare in adults, but common in infants under the age of two. It is sometimes difficult to pinpoint the specific substances responsible for a food allergy because reactions are often delayed or may be caused by food additives. However, approximately 85% of food allergies are caused by proteins in cow's milk, eggs, peanuts, wheat, or soybeans. Other common food allergens include berries, shellfish, corn, beans, and some food colourings. The classic symptoms of food allergy include coughing, skin rashes, itchy eyes, stomach cramps, diarrhoea, and nausea. In more severe cases, there may be vomiting, swelling of the face and tongue, and respiratory congestion, as well as dizziness, sweating, and faintness.
Drug allergies: The most common drug allergy is to antibiotics in the penicillin family. Allergies have been seen to many types of antibiotics - especially sulfa drugs. Sulfa is also found in drugs other than antibiotics, such as in arthritis drugs. Allergies have been seen in many other types of drugs, including dyes injected into blood vessels for X-rays. Reactions to aspirin are very common too.
Insect sting allergies: Some studies speculate that people who have other allergies - food, drug, or respiratory - may be more susceptible to insect sting allergies. Venom in stings of bees, wasps and hornets is a common allergen.