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WHAT ARE ALLERGIES?

An allergy is an immune response or reaction to substances that are usually not harmful.

Causes

 

Allergies are very common. Both genes and environment play a role. If both your parents have allergies, there is a good chance that you have them, too.

The immune system normally protects the body against harmful substances, such as bacteria and viruses. It also reacts to foreign substances called allergens. These are usually harmless and in most people do not cause a problem.

Allergic reactions

In a person with allergies, the immune response is oversensitive. When it recognizes an allergen, the immune system launches a response. Chemicals such as histamines are released. These chemicals cause allergy symptoms.

Common allergens include:

  • Drugs
  • Dust
  • Food
  • Insect venom
  • Mold
  • Pet and other animal dander
  • Pollen

Some people have allergy-like reactions to hot or cold temperatures, sunlight, or other environmental triggers. Sometimes, friction (rubbing or roughly stroking the skin) will cause symptoms.

Allergies may make certain medical conditions, such as sinus problems, eczema, and asthma, worse.

Symptoms

Mostly, the part of the body the allergen touches affects what symptoms you develop. For example:

At times, an allergy can trigger a response that involves the entire body.

Hives (urticaria) on the chest

References

Chiriac AM, Bousquet J, Demoly P. In vivo methods for the study and diagnosis of allergy. In: Adkinson NF Jr, Bochner BS, Burks AW, et al, eds. Middleton's Allergy: Principles and Practice. 8th ed. Philadelphia, PA: Elsevier Saunders; 2014:chap 70.

Wasserman SI. Approach to the patient with allergic or immunologic disease. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 25th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 249.

 
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