Allergies and Immunology

What are they?
Allergies are hypersensitivities, overreactions of the immune system to substances that do not cause reactions in most people. Hypersensitivities are grouped into four types, I through IV. These classifications are based, to some extent, on what parts of the immune system are activated and how long it takes for a reaction to occur.

The two types of hypersensitivities commonly associated with the term “allergies” are type I immediate hypersensitivities, in which antigens (allergens, foreign substances) combine with specific IgE (immunoglobulin E) antibodies to cause local and sometimes systemic reactions – usually within minutes; and type IV delayed hypersensitivities, reactions caused by the interactions of antigens with specific sensitized T lymphocytes instead of antibodies.

Type I hypersensitivities affect mainly the respiratory and gastrointestinal systems and the skin. They occur most frequently in those with a predisposition to allergies (those whose parents have allergies are more likely to have them themselves, although not necessarily to the same substances). The first time a predisposed person is exposed to a potential allergen, they will not have a major reaction; instead, they will create a specific IgE antibody and become “sensitized.”

The IgE antibody produced then attaches itself to mast cells, specialized cells in the tissues, and basophils (a type of white blood cell) in the blood stream. This action primes the immune system. During subsequent exposures to the allergen, that specific bound IgE identifies the intruder, attaches to it, and triggers the release of chemicals, including histamine, causing allergic symptoms that start in the mouth, nose, or on the skin, wherever the allergen was introduced.

On the skin, an acute type I allergic reaction causes hives, dermatitis, and itching, while chronically, the allergy may cause atopic dermatitis and eczema. In the respiratory tract, the acute allergic reaction causes coughing, nasal congestion, sneezing, throat tightness, and, chronically, asthma. It can also cause red itchy eyes. Acute allergic reactions in the gastrointestinal system start in the mouth with tingling, itching, a metallic taste, and swelling of the tongue and throat, followed by abdominal pain, muscle spasms, vomiting and diarrhea, chronically leading to a variety of gastrointestinal problems.

Any severe acute allergic reaction has the potential to be life threatening, causing anaphylaxis, a multi-organ reaction that can start with agitation, a feeling of “impending doom,” pale skin due to low blood pressure, and/or loss of consciousness (fainting). Anaphylaxis can be fatal without the rapid administration of an epinephrine (adrenaline) injection. Type I allergic reactions can be variable in severity, one time causing hives, the next time anaphylaxis.

Type I allergies can be to just about anything: foods, plants (pollens, weeds, grasses, etc), insect venoms, animal dander (such as cat and dog), dust mites, mold spores, occupational substances (latex), and drugs (such as penicillin). There can also be cross-reactions, where someone allergic to ragweed, for instance, may also react to melons (watermelon or cantaloupe) and banana. The most common food-related causes of severe anaphylactic reactions are peanuts, tree nuts (such as walnuts), and shellfish.

Type IV delayed hypersensitivity reactions are most often skin reactions. Common examples include reactions to metal and jewelry. They occur when an antigen interacts with specific sensitized T lymphocytes. The lymphocytes release inflammatory and toxic substances, which attract other white blood cells to the exposure site, resulting in tissue injury. No immune system “priming” is necessary; patients can have a type IV reaction with the first exposure. Type IV hypersensitivity is usually a reaction (redness, swelling, hardening of the skin, rash, dermatitis) observed at the exposure site hours to days after exposure.

Related Page On This Site
Tests: Allergy testing
Conditions: Asthma
In the News: FDA Clears New Breath Test for Monitoring Asthma (2003)