What is ALLERGY?

Allergy is a form of immune response that can be inherited and then the allergic individual can be "sensitized" or allergic to common things in the environment including pollens, danders and moulds (fungi).
When the "sensitized " person is exposed to those things things (allergens) in their environment , they may develop an allergic response that can involve nasal and eye symptoms (such as itchy eyes, runny nose, sneezing) or asthma symptoms (cough, wheeze and/or shortness of breath), eczema and rash.
If a child becomes allergic in the first few years of life, it is usually to a food protein. After age 2 to 3 years, they move away from foods towards aeroallergens (allergens in the air) often animals even when there are no pets at home. This is called the allergic march.
Because atopic dermatitis usually develops in infancy even in the first few months after birth, the allergic sensitivities are to foods but later they develop sensitivity to aeroallergens similar to other allergic children
In food allergic children, 6 or 7 foods create the majority of problems with milk and egg being the commonest but peanut not far behind.
About 20 - 30% of the population has an allergic tendency. When a person becomes sensitized to an allergen, the allergen is first processed by a group of cells leading to lymphoid cells (T lymphocytes of the Th2 class) that are sensitized i.e. recognize the allergen as foreign material. These cells are a different group than the T cells that recognize more classical foreign elements such as bacteria and viruses. The T cells form factors called cytokines and some of them interact with another class of lymphocytes (B cells) that make IgE antibody to the allergen. This is the protein that governs many allergic reactions.
This IgE can sit on the surface of other classes of cells e.g. mast cells and basophils. Then when an allergen enters and binds with the IgE sitting on the surface of the mast cell, a process is triggered such that the mast cells release molecules that they carry into their surrounding milieu. The most widely known of these molecules is called histamine. But there are others. Anti-histamines are commonly used to prevent some of the symptoms of allergy.
This release of histamine by allergens is the basis for the skin testing used in the allergists' office. When the allergen is placed on the skin and pricked, there is release of histamine into the surrounding skin which leads to dilatation of blood vessels (redness), oozing of fluid from the blood into the tissue (the wheal) and itching. This reaction occurs within 15 - 20 minutes ("immediate phase reaction").

The allergy Skin Test.
However the reaction with allergen is much more complex than that. Dr. Jerry Dolovich of McMaster University in the early 70s first recognized that there was also a "late phase reaction" occuring a few hour later and involving cells such as the eosinophil. Since then we have come to understand that cellular activity especially involving the eosinophil is a very important part of the allergic immune response and plays a very important role in such diseases as atopic asthma and atopic dermatitis. The eosinophils can be controlled directly by the sensitized Th2 cells.
There is currently a lot of research being done on the prevention of allergy sensitization (primary control) especially in view of the fact that for unknown reasons allergy seems to have doubled in the population in the last twenty years. Similarly secondary allergy control (control after the person has already become sensitized) is being investigated. It is expected that in the next number of years, many new methods for allergy prevention and treatment will come forward.

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