What is Hypersensitivity reaction – Type I

Hypersensitivity means that the body responds to a particular substance (called allergens) in an exaggerated fashion, where it does not happen in normal circumstances. There are 4 types of hypersensitivity reaction, type I, II, III and IV. They are different in terms of the disease manifestation and pathological processes. However, only type I hypersensitivity reaction will be discussed here.
It is an immediate immune reaction, ie it happens immediately after exposure to the particular substance.
Type I hypersensitivity reaction can occur in many parts of the body, but generally they include:


Hypersensitivity reaction - Type I

Statistics on Hypersensitivity reaction – Type I

Type I hypersensitivity diseases are extremely common. The common ones are allergic rhinitis (hay fever), allergic conjunctivitis, and asthma.
The number of cases of type I hypersensitivity diseases is increasing worldwide over time.

Risk Factors for Hypersensitivity reaction – Type I

Depending on the manifestations of different type I hypersensitivity diseases, there are different risk factors as well.
Generally, having a type I hypersensitivity disease increases the risk of having another.
Other risk factors are detailed as below:

  • gender: there are unexplained differences in relation to gender in allergic diseases. Some are more common in males than females such as asthma, while allergic eczema is more common in females.
  • age: generally allergic symptoms tend to diminish with increasing age. This applies to conditions such as allergic rhinitis and allergic conjunctivitis. However conditions like anaphylaxis and angioedema do not have any age preference.


Progression of Hypersensitivity reaction – Type I

Type I hypersensitivity is characterized by Ig E mediated reaction. Ig E are antibodies found in body secretion (such as tears, nose secretion) that are responsible for allergic reaction. Upon exposure to allergens (such as pollens in the air, or certain drugs used in hospital), the Ig E antibodies bind to the allergens in these bodily secretions such as tears. Such binding will eventually lead to the release of histamines (a biochemical compound in the body responsible for allergic symptoms). Fiinally the patients will have symptoms such as swelling, increased secretion discharge, redness, etc.
Ig E mediated hypersensitivity reactions are also known as immediate hypersensitivity reactions. This is because the response is immediately noticeable upon exposure to allergens.

How is Hypersensitivity reaction – Type I Diagnosed?

All of the type I hypersensitivity diseases do not need any investigations in general. The diagnoses are made using clinical history and examination, even in life-threatening situations likeanaphylaxis and angioedema.

Prognosis of Hypersensitivity reaction – Type I

Type I hypersensitivity is short-lived in terms of its reaction, however due to different diseases that affect the body, the prognosis can be different as well. Common diseases such as allergic rhinitis (hay fever), allergic conjunctivitis and bronchial asthma rarely cause deaths except in severe asthma. However angioedema and anaphylaxis can potentially cause death if not treated immediately.
Type I allergic hypersensitivity diseases do not have a cure. The aim of the treatment is to control the symptoms.

How is Hypersensitivity reaction – Type I Treated?

There is no cure for these diseases, the treatment aims at symptom control only. Although type I hypersensitivity diseases can cause significant inconvenience in terms of their signs and symptoms, with good treatment plan the most of the diseases can be well-controlled.
Depending on the severity of the allergic reaction, different treatment approaches are applied. Severe allergic reaction such as anaphylaxis and angioedema needs prompt medical attention as death may ensue within minutes. Therefore the treatment plan will be aggressive resuscitation with appropriate supportive treatment. However diseases that are not life-threatening may be treated with differently. Treatment options, either given alone or in combination, include the following:

  • allergen avoidance: prevention and avoidance of possible triggers are the mainstay of the treatment. This include limiting outdoor exposure, shutting windows in the house and cars, using air conditioning system indoor, frequent washing of bedding, and frequent cleaning of the home environment. If certain allergens are suspected like pets, it will be appropriate to remove the pets altogether from the household. Other measures include removal of old curtains, carpets and furniture.


  • antihistamines: this can either be used by applying on the site of involvement or taken by mouth. Antihistamines include fexofenadine, loratadine and cetirizine.


  • mast cell stabilisers: because mast cell (a type of white blood cell) plays a major role in allergic reactions, the aim of these drugs is to block the action of mast cells. Examples of these group of drugs include sodium cromoglycate and lodoxamide tromethamine. These can be applied to the site of involvement or taken by mouth.


  • steroids: these include prednisolone, dexamethasone, etc. They can be applied on the site of involvement (eg skin) or taken by mouth. Steroids are especially useful in moderate to severe allergic eczema.

Other medications that are not commonly used include:

  • cyclosporine
  • immunotherapy: this is gradual sensitisation of the individual towards known antigen.


Hypersensitivity reaction – Type I References

  1. Braunwald E et al. Harrison’s principles of internal medicine. 15th ed. New York; McGraw-Hill; 2001.
  2. Middleton E Jr, Reed CE, Ellis EF, et al, eds: Allergy: Principles and Practice. 5th ed. St. Louis, Mo: Mosby-Year Book; 1998.
  3. The Merck Manual: Disorders with Type I Hypersensitivity Reactions [online]. 2005. [Cited 2005 October 8th]. Available from: http://www.merck.com/mrkshared/mmanual/section12/chapter148/148b.jsp
  4. Up to Date: Allergic eye disease [online]. 2005. [Cited 2005 October 8th]. Available from: URL: http://www.utdol.com/application/topic.asp?file=allergy/6123&type=A&selectedTitle=1~5
  5. Up to Date: Anaphylaxis [online]. 2005. [Cited 2005 October 8th]. Available from: URL: http://www.utdol.com/application/topic.asp?file=cc_medi/5448&type=A&selectedTitle=1~59
  6. Up to Date: Clinical manifestations and evaluation of allergic rhinitis (rhinosinusitis) [online]. 2005. [Cited 2005 October 8th]. Available from: URL: http://www.utdol.com/application/topic.asp?file=allergy/6500