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The immune system is an intricately designed defense mechanism. It acts to neutralise infectious bacteria, viruses and generally protect the body from disease.

However, when the immune system is too sensitive, it can react to normally harmless substances in the environment. This is an allergic reaction.

The substances that cause an allergic reaction are called allergens. These can be biological (pollens, dust, bacteria), chemical (rubber, nickel, isocyanates in spray paint) or physical (heat, light, electromagnetic radiation).



So what’s going on in the body?

The body’s first response to an allergen is to release antibodies. These antibodies stimulate cells in the blood to release histamine.

Histamine affects lots of different body tissues. As we saw in the post Peptics Ulcers and Medical Treatment, there are histamine receptors in the stomach that release gastric acid. However, there are also histamine receptors in the skin, lungs, sinuses, and certain blood cells. This means that there are many possible responses to release of histamine:

  • Skin: a rash or itch.
  • Lungs: narrowing of airways. This can cause wheezing and difficulty breathing.
  • Sinuses: increased mucous production. This can cause a runny nose or productive cough.
  • Blood: widens blood vessels. This causes a rapid drop in blood pressure which can lead to shock.

Some of these reactions can be useful when fighting off a real infection and trying to get bacteria out of the body. For example, the widened blood vessels allow faster transport of white blood cells to attack a source of infection.

In addition to this, increased mucous production protects areas such as the lungs from bacterial invasion. Mucous is used to protect internal body tissue: in the lungs, it is a protective layer between bacteria-containing air and body tissue. Increased mucous production is intended to trap and expel bacteria from the body (when it is coughed out).


How serious are allergies?

The severity of an allergic reaction ranges from mild, such as hayfever, to severe. Anaphylaxis is an extreme all-body allergic reaction with symptoms such as difficulty breathing, itchiness, nausea and dizziness. It can have fatal consequences if not treated immediately by a professional.

If you come across someone who is in anaphylactic shock, the National US Library of Medicine advises following these steps: (http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001847)

  1. Call 111 (in NZ)
  2. Calm and reassure the person
  3. If the allergic reaction is from a bee sting, carefully scrape the stinger off the skin with a credit card. DO NOT use tweezers: squeezing the stinger releases more venom.
  4.  If the person has emergency medicine on hand, help the person to take or inject it. Avoid oral medication such as pills if the person is having difficulty breathing.
  5. If there is no concern about suspected head/neck/back/leg injury, take these steps to prevent shock:
  • Have the person lie down
  • Raise the person’s feet 30cm or so off the ground
  • Cover them with a coat or blanket


Treating allergic reactions

Severe reactions: Adrenaline is the body’s natural histamine inhibitor, blocking the histamine receptors. An adrenaline dose is often carried by a person who has anaphylaxis. When given to a person in anaphylactic shock, the adrenaline should block the symptoms of an allergic reaction. This can be life-saving.

Mild reactions: Anti-histamines are often prescribed for people with mild allergies. These drugs also block histamine receptors, but particularly target the receptors in the skin and sinuses to prevent rashes and a runny nose. 


Doctors may use ‘allergy shots’ to prevent allergic reactions. The idea is to inject your body with very small doses of whatever it is you are allergic to. This should slowly change your body’s reaction to this allergen. The process is a slow one and may take up to 5 years.





The American Environmental Health Foundation website at http://www.aehf.com/articles/article36.html



Article about anti-histamines: J Allergy Clin Immunol. 1997 Feb;99(2):S798-806. Available at http://www.ncbi.nlm.nih.gov/pubmed/9042073