Allergy, Allergens and Allergic Reactions
Allergy is an acquired hypersensitivity to a normally benign substance. The term is derived from the Greek altos, meaning “altered,” and ergia, “reactivity.” An allergen is any substance that provokes an allergic reaction.
Allergic reactions are a unique form of auto-immune disease, possibly evolved from a genetic mechanism that protected the body against worms and parasites.
The allergens may in some way mimic these invading organisms. It appears that individuals inherit a genetic predisposition to develop a particular allergy, though the mode of transmission is unclear. It is likely to be polygenic and/or mult-factorial, though single genes may be involved as well in some allergies, inherited in an autosomal dominant manner.
Common allergens include pollen, dust, hair, fur, feathers, scales, wool, chemicals, drugs, insect bites, and such foods as eggs, chocolate, milk, wheat, tomatoes, nuts, citrus fruits, shellfish, oatmeal, sulfite preservatives and potatoes.
Evidence also indicates deer can make some hunters sneeze, cockroaches can prompt asthma attacks in asthmatics, physicians can develop an allergy to latex and some individuals are literally allergic to exercise. Concern over peanut allergies has provoked efforts to ban peanut butter in schools.
The prevalence of allergies is unknown, though it is thougth potentially to represent that largest population of chronically diseased individuals in the United States.
Common allergic conditions include hay fever (vasomotor or allergic rhinitis), bronchial asthma, eczema (a skin condition) and hives (urticaria).
An estimated 40 million Americans can have hay fever, 9 million asthma, from 10 million to 20 million have had occasional hives and an untold number have food allergies.
Because of the prevalence of these disorders and their morbidity and economic impact (consider absenteeism in schools and workplace related to asthma alone) much effort has been made to identify the genes involved, which may eventually allow for a pharmocogenetic approach to treatment.
Symptoms of the reaction commonly involve the respiratory tract or the skin. In the respiratory tract this usually takes the form of congestion, runny nose, watery eyes, sneezing and breathing difficulty caused by swelling and constriction of the bronchial tubes. Cutaneous involvement includes itching, rashes and lesions.
Allergies usually develop following a number of exposures to a given substance, after enough antibodies are produced to trigger the response to the allergen. The antibodies are a class of immunoglobulins (IgE Opens in new window), with a specific form for each allergen.
When allergens are present, the IgE antibodies attach themselves to cells in the lining of the nose and bronchial passages, where they bind to the allergens. This triggers the cells to eject histamine or histamine-like substances, causing an irritation accompanied by sneezing, itching and watery eyes. Severe reactions can cause death by blockage of air passages or a precipitous drop in blood pressure.
Genes Opens in new window have been found that may increase susceptibility to allergies that result in asthma and hay fever attacks. Individuals with one such gene often have high levels of IgE in their blood, an indicator of allergic overreaction.
Another implicated gene codes for production of interleukin 4 Opens in new window, a protein that regulates the body’s production of IgE. Individuals with a mutated copy of this gene are 10 times likelier than the general population to have an allergy.
Atopic eczema Opens in new window, which is often associated with asthma and other allegic phenomena, is extremely common, affecting 0.7% of the population. Some believe that it may even be determined by an autosomal dominant gene with highly variable expression.
The risk of developing some allergic problem where one parent is affected approaches 50% and is slightly higher when both parents are affected.
- Adapted from The Encyclopedia of Genetic Disorders and Birth Defects By James Wynbrandt, Mark D. Ludman