The term "allergies" includes a wide variety of conditions, ranging from irritating hay fever to life-threatening anaphylaxis. All of these reactions, however, share much in common. The immune system is made up of a large number of cells that protect against foreign infections.
Most allergies are due to an abnormal reaction of the acquired immune system towards foreign antigens. However, a few reactions occur due to abnormal reactivity of the innate immune system. These reactions are known as "pseudoallergies."
The Immune System
The immune system can be classified broadly as the natural, or innate immune system, and the specific, or acquired immune system. The natural immune system includes a variety of defensive barriers to infections that are found in all people. Skin, for example, protects the body against foreign invaders, as does the acidity of the stomach. Certain cells of the immune system act regardless of the specific nature of the invader in question. This includes neutrophils and macrophages.
The acquired immune system, on the other hand, is specifically targeted to attack certain microbes. The most important cells of the immune system are known as T cells, which organize and coordinate the other classes of immune cells in a reaction. T cells can activate B cells, a type of cell which produces antibodies that neutralize foreign antigens. T cells can also activate macrophages, cells which can migrate to a site of infection, take up the antigen, and break it down.
Allergies and Pseudoallergies
Most types of allergies involve the acquired immune system. In these reactions, B cells produce an unusual antibody, known as IgE. IgE binds to mast cells; when the specific allergen binds to IgE, the mast cell releases histamine and a variety of other substances which trigger the allergic response.
Some reactions do not involve the specific immune system. Instead, certain agents may circumvent the cells of the specific system and cause an inflammation by directly activating cells of the natural immune system, thereby triggering an allergic response.
Contrast Allergies
Contrast agents are used by radiologists for some types of computerized tomorography, magnetic resonance imaging or angiography. These agents allow for increased delineation of internal anatomy. These contrast agents may contain iodine or gadolinium. Sometimes, these agents can directly activate mast cells to stimulate an allergic reaction. The symptoms of these pseudoallergies include hives, throat tightening, constriction of the airways or even anaphylactic shock. No research has been able to identify the specific immune trigger for these reactions, leading to the belief that these are not mediated by the specific immune system.
Aspirin-Induced Asthma
While most types of asthma are atopic, that is, induced by the specific immune system, other kinds of asthma may occur in relation to specific medication. Aspirin-induced asthma is the most well known of these. Instead of having a specific allergy to aspirin, the medication can block the formation of certain muscle-relaxing molecules, called prostaglandins. This can lead to uncontrolled muscle constriction in the airways, resulting in a reaction that mimics an asthma attack. Aspirin-induced asthma can be treated with bronchodilators, steroids and possibly desensitization therapy.
Morphine Pseudoanaphylaxis
Morphine, a medication used to treat severe pain, can sometimes cause a severe pseudoanaphylactic reaction. As with true anaphylactic reactions, symptoms include chest tightness, difficulty breathing, a fall in blood pressure, weak pulse and irregular heartbeat. The drug is thought to act by directly activating mast cells rather than by acting on the specific immune system. This severe reaction requires emergency treatment.
References
- "Annals of Allergy, Asthma & Immunology"; The Role of Epigenetics in the Developmental Origins of Allergic Disease; Michelle L. North, et al.; May 2011
- VHL Family Alliance: Allergies to Contrast Media; Peter L. Choyke, M.D.; December 2004
- University of California San Francisco: Iodine Allergy and Contrast Administration
- "The Journal of Allergy and Clinical Immunology"; Aspirin-Induced Asthma: Advances in Pathogenesis, Diagnosis and Management; A. Szczeklik, et al.; May 2003
- Medline Plus: Anaphylaxis; 5/2/2010
- "Lancet"; An Overview of the Immune System; Jacqueline Parkin, et al.; June 2001
