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Tuesday, April 14, 2009

The Four Types of Allergies

True allergies are reactions of the IgE antibody system. But there are other antibody systems in the body too and they can produce their own sets of usually different symptoms, creating much confusion. The two types of reactions are sometimes both lumped together as allergies, but they're better thought of separately as allergies and hypersensitivities respectively.

I have a page on my web site, Lactose Intolerance versus Milk Allergy, talking about this. I've posted about it before as well, in The Different Types of Dairy Allergy.

I thought about those when I ran across a column by Dr. Jeff Hersh on the Norwood, MA Daily News Transcript website. He answers a pertinent question from a reader.

Q: My daughter had a blood test which showed she has a delayed IgG egg allergy rather than an immediate IgE (immunoglobulin E) allergy. Can you tell me what this really means?

The whole column is too long to quote here but has a good number of technical details in it that may help people, especially parents, understand allergies better. A few excerpts:
1. IgE helps defend against parasites. It also binds to allergens (things that trigger allergies) to trigger histamine release from mast cells, although how this helps defend the body is not understood.

2. IgG and IgM help "tag" many types of infections, with IgM forming during an acute infection and IgG serving as a "memory" to fight a future similar infection. ...

The types of hypersensitivity are classified according to the part of the immune system causing the problem (although other classification systems exist):

Type 1 is an immediate reaction (usually within minutes to hours) due to IgE and is very common, affecting 50 million Americans. It may be due to foods, environmental allergens (pollen, grasses, etc.), insect stings, medications or many other causes. Typical symptoms include rashes (such as hives), gastro-intestinal symptoms (such as nausea, vomiting and/or diarrhea), swelling (angioedema) and/or respiratory symptoms (such as runny nose, nasal congestion, cough, or even asthma). An estimated 500 people die each year from anaphylaxis, a life-threatening hypersensitivity reaction where symptoms are so severe breathing may be compromised and blood pressure can drop.

Type 2 is due to IgM, IgG and/or complement, and includes conditions where the body's immune system attacks normal body cells such as in autoimmune hemolytic anemia (the red blood cells are attacked), Goodpasture's disease or others.

Type 3 is due to immune complex, IgG and/or complement and can lead to serum sickness or other conditions. It may also lead to allergy symptoms similar to type 1, however with delayed (usually within hours to days) onset.

Type 4 is also delayed onset, but is due to T-cells. It is responsible for contact dermatitis (such as poison ivy), as well as other conditions.

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