Allergic Disorders

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Before reviewing the topics on the left, it will be helpful to understand a few important concepts about allergy.  An explanation of these concepts follows a brief summary.

Links to general allergy concepts on this page:

Allergy   Immune Globulin E (IgE)   Sensitivity  Allergic Reaction   Inflammation

Summary: Allergy is an interaction between the immune system and a substance foreign to the body. There are two important effects of allergy on the body. One is the classic "allergic reaction" that can cause sudden tissue swelling, itching, sneezing, increased mucous production, and in the lungs, wheezing and shortness of breath. In food allergic reactions, the intestinal tract can also be acutely inflamed and cause nausea, vomiting and diarrhea. In a "whole body" allergic reaction, including anaphylaxis, low blood pressure and shock can occur.  The other part of the "allergic response"  is persistent inflammation that is important in nasal allergy, asthma and eczema. In allergy of the airway, inflammation can occur and cause some symptoms without the sneezing, itching and wheezing.   Diagnosis and effective treatment of allergic disorders requires consideration of both of these effects.

Allergy:    When reading about or discussing allergy, it is important to know how the term is defined. In Webster's Dictionary, allergy is defined as  "excessively sensitive to a specific substance or condition". This is a very broad definition which implies that anything that adversely affects our body can be considered an allergy. The medical definition however is much more restricted; an interaction between a foreign (outside of the body) substance and the immune system which leads to a harmful event. This "foreign substance" or protein structure is termed an "allergen". Common allergens include aeroallergens such as pollens, dust mite and animal dander proteins, and food allergens such as peanut and shellfish.    

Immune globulin E ( IgE ):  The first immune system component to interact with an allergen,  in most of the disorders related to allergy-immunology is the "allergic antibody" immune globulin E, or IgE. Antibodies such as IgE are "specific" in that they can attach to only one type of protein (allergen). Thus, some individuals have allergy only to cat and mold allergens, while others may have allergy to dog and pollen allergens and not to cat or mold. This "specificity"  is an important characteristic of the immune system and it contributes to the great diversity of clinical expression in allergy and asthma. This is why diagnosis and treatment plans must be individualized for each child and adult with an allergic condition. In tissues,  IgE antibodies are attached to mast cells, which contain histamine. There may be thousands of the same copy of IgE antibody, along with copies of antibodies that bind to other allergens. 

Sensitivity: A harmful  reaction can also occur after exposure to various irritants in our environment such as exhaust and solvent fumes, and strong odors. Bothersome symptoms such as eye and nasal irritation, cough and shortness of breath are often considered an "allergic reaction", but in such cases, symptoms do not occur because of an interaction with the immune system, but rather, because of a direct irritant effect on the tissues of the body.  Therefore, one is not "allergic",  according to the more restrictive definition, to such irritants,  but rather "sensitive" to an offending foreign substance. It is true that those with allergy and/or asthma are more sensitive to odors, fumes and other irritants in our environment.  The distinction between allergy and sensitivity  is important in regards to diagnosis and treatment of allergic - immunologic disorders. This topic in relation to food allergy and sensitivity is continued on the page   Food Allergy.           

Allergic Reaction: An allergic reaction involving IgE occurs within minutes after binding of an allergen to IgE antibody located on mast cells on or within mucous membranes or within certain tissues of the body.  The IgE antibody's structure consists of a body and two arms with "hands" (binding sites) that  hold on to only one specific allergen. The "foot" of the IgE antibody is attached to the mast cell which is packed full of histamine granules. These antibodies may be attached to the mast cells for weeks to months, and unless there is an allergen to bind to, there are no symptoms to indicate allergy. Once an allergen comes along, such as alder tree pollen in the spring time,  symptoms start to occur in those allergic persons who have IgE antibody specific for alder pollen.

When allergen binds to its corresponding antibody bound to a mast cell (actually bridging two copies of the same antibody), a signal is sent to the inside of the cell causing the cell membrane to burst open, releasing histamine granules in an explosive like process.  Histamine then diffuses out into adjacent tissue and interacts with nearby blood vessels, nerve cells, and, in the nose and lungs, mucous glands. Blood vessels then enlarge and leak fluid and tissues swell, and itching occurs from interaction with nerve cells.  In the nose, sneezing and increased mucous production will be noted. In the lungs, increase mucous secretion and airway narrowing occurs if one has asthma. In order for histamine to cause allergic symptoms, it must bind to specific histamine receptors on the above tissues. This receptor can be "occupied" by a medication called an antihistamine, blocking the action of histamine leading to reduced allergic symptoms.

Inflammation: In addition to release of histamine, mast cells release a number of molecules that act like "magnets" for immune cells that circulate in the blood stream. Allergic and non allergic individuals have the same immune cells, but those with allergy have more of certain types of cells, such as mast cells and another important cell, the eosinophil. These, and certain other immune cells, as they travel through the microscopic blood vessels in tissues where mast cells are being "activated" by allergen,  move out of the vessel and into the tissues, building up over time and leading to persistent allergic and/or asthmatic symptoms. This part of the allergic reaction can occur on a near continuous basis as long as there is allergen exposure, such as animal dander, mold or dust mite exposure. It may occur even without the acute allergic reaction associated with histamine release.  Although this process may be "silent", lacking the sneezing, wheezing, and itching of an acute allergic reaction, it is the most important aspect of allergic-immunologic disorders associated with frequent and often, difficult to control symptoms seen in chronic allergy and asthma. This process does not respond to treatment such as antihistamines, or quick relief -  bronchodilator therapy for asthma.