What is an allergy?
An allergy is an abnormal sensitivity to an allergen that is inhaled, eaten or touched
that most other people can tolerate with no trouble. Allergies are over-reactions
by someone to substances their immune system considers harmful even if they are
not. A normal reaction should be the development of immunity. Our immune system
is there to defend us from any aggression or invasion of foreign substances.
Its means of defense are sophisticated and complex. They consist of producing antibodies,
which are our body's "defenders". Their job is to destroy intruders, or
so-called antigens. The antibodies involved in allergies are known as IgE
(Immunoglobulin E). Everyone makes some IgE – only the people with genetic
predisposition towards allergies make large quantities. Whereas, IgG antibodies
rid the body of infections, like a strep throat & viruses, IgE antibodies have
misdirected attacks against pollens, dustmite, dander, and moulds. These immunological
(involving the immune system) reactions should be theoretically beneficial. But,
in some cases, our immune system goes overboard and "overdoes it", or
loses control, reacting to harmless substances that pose no danger or threat to
This over-reaction and loss of control by the immune system is seen in people with
allergies, where the immune system launches attack against a perfectly harmless
substance, such as grass pollen, cat dander or peanut or penicillin. The harmful
end results of these attacks are called immediate hypersensitivity or allergy. The
antigen responsible for an allergic reaction is called an allergen. The other main
components to allergies are mast cells, which contain chemicals like histamine.
The IgE Antibodies sit on the surface of the mast cells. A mast
cell has about 1,000 histamine containing packets (granules) in its cytoplasm and
on its surface are between 100,000 and 1 million receptors for IgE. When the IgE
encounters allergen, it triggers the mast cell to release granules from its cytoplasm.
Those granules contain histamine and other chemicals. These mediators that are released
then interact with the lining of the site involved (skin, nose, eyes, lung etc),
causing the allergic symptoms.
The word allergy comes from the Greek allos, meaning other. It was first used in
1906 to refer to "altered reaction" in the body's immune system. Since
then the term allergy has been used to describe a host of conditions most of which
have nothing to do with the immune system. For someone to have an allergic reaction,
they have to be sensitized to the allergen. Being sensitized means that the immune
system has been in contact with an antigen, that it has committed it to memory and
has produced specific antibodies against it. At a later contact, it will recognize
the antigen and immediately react against it. Our immune system has a good memory.
From the first time the immune system comes in contact with a foreign substance,
it learns how to recognize it and memorizes it. That is why someone suffering from
hay fever will react every time he comes in contact with that specific type of pollen
that has been memorized by his immune system.
True allergic reactions occur within minutes to an hour of contact with the allergen.
This differentiates them from pseudo-allergic reactions, which do not involve the
immune system and can occur up to 48 hours after contact with the offending substance.
Some people use the term intolerance to refer to some of the non-immunologic adverse
reactions to foods.
What are some common Allergens?
There are two types of environmental allergens:
Seasonal: These are mainly outdoor allergens and include
from grasses, trees and weeds, and also some moulds.
Perennial: These are usually indoor allergens, and cause symptoms
all year round. The commonest is the house dust mite. These
reside in nests such as carpets, bedding and upholstered furniture. House dust mites
proliferate in humid homes. They are the commonest triggers for perennial rhinitis
and allergic asthma.
Other perennial allergens include domestic pets, especially cats.
The actual allergen is a protein component of dander, the shed scales of the skin.
The same allergens are also found in the animal"s saliva and urine. Hair on
fur can also collect other allergens, such as pollens, moulds and dust mites.
Only about six foods account for more than 90% of all confirmed food allergies.
These are milk, eggs, peanut & other nuts, wheat, soy and fish.
Aspirin & other anti-inflammatory drugs and antibiotics like penicillin account
for most of the allergic reactions due to drugs.
Each year about 40 people die in the USA from Anaphylaxis to insect (bee, wasp &
fire ants venom) sting.
Over the last 15 years immediate hypersensitivity (IgE-mediated allergy) to latex
has been increasingly recognized as a rapidly emerging public health problem. Latex
is widely used in the manufacture of medical devices (gloves, catheters, anaesthetic
masks, and dental dams) as well as a variety of everyday articles. Reactions from
latex range from annoying skin rashes to life-threatening anaphylaxis.
Is allergy inherited?
There are three classic kinds of atopy:
(eczema), hay fever (seasonal allergic
rhinitis) and allergic asthma.
Most allergy sufferers are atopic. Atopy means that the individual has a genetic
tendency to develop allergies. In most developed countries up to 40% of the population
will be atopic. Even only one-half of them will develop allergic symptoms. Atopic
individuals are genetically prone to manufacture antibodies of the IgE class. These
IgE (immunoglobulin E) antibodies tend to react against allergens present in the
environment (pollen, cat dander, moulds, house dust mite). Atopy is therefore easily
diagnosed in an individual who has a personal or immediate family history of allergies
and on doing skin prick test they react to one or more of the common environmental
These individuals with the genetic predisposition must be exposed to the allergen
either in their environment or in their diet. So allergy is partly inherited and
If one of your parents has a respiratory allergy, you have a 30 to 50 percent chance
of developing one, though not necessarily the same allergy. If both of your parents
have respiratory allergies, the risk that you will develop an allergy goes up to
Do children outgrow allergies?
Sometimes. Food allergies are more likely to be outgrown than inhalant allergies.
Food allergy is much more common in children than adults, because up to 80% of infants
will outgrow their allergy to milk and eggs by the age of five.
How is allergy diagnosed?
A good medical history is usually the basis for the diagnosis of any allergy. Knowledge
of the family history of allergic problems is very helpful. However, it may take
some good detective work to figure out exactly what the allergen is.
Skin Prick Tests
To confirm which allergen is responsible, skin prick tests may be recommended using
extracts from allergens such as house dust mites, pollens, or moulds commonly found
in the local area. A diluted extract of each kind of allergen is pricked into the
skin of the forearm or on the back.
With a positive reaction, a small, raised, reddened area with surrounding flush
(called a wheal and flare) will appear at the test site. The results are measured
after 15 minutes. The size of the wheal can provide the physician with an important
diagnostic clue, but a positive reaction does not prove that a particular allergen
is the cause of the patient"s symptoms. It proves that IgE antibody to that
specific allergen is present in the skin.
Diagnostic tests can be done using a blood sample from the patient to detect levels
of IgE antibody to a particular allergen. One such blood test is called the RAST
(radioallergosorbent test), which can be performed when the entire body is covered
with eczema or if the patient is on antihistamines, which will interfere with skin
prick test results.
Double-Blind, Food Challenge
If diagnosis of food sensitivity remain in doubt after the patient has used diet
diaries, eliminated certain foods, and undergone allergy tests, the allergist may
recommend "blinded" food and/or food additive "challenges" in
the office or hospital under close observation. Usually, the suspected food or a
neutral food, called a placebo, is fed in opaque colorless capsules, or as a hypoallergenic
slush or pudding so neither the patient nor the doctor knows whether the suspected
food or placebo is being eaten. This is called "double-blind" challenge
and when properly performed is helpful in establishing the cause-and-effect relationship
between a food and an allergy symptom. This is considered by some to be the gold
standard for diagnosing any adverse reaction to foods.
How are allergies treated?
The diagnosis should ideally be confirmed, to determine the exact source of the
allergy, before commencing treatment. The best way to treat allergies is to avoid
exposure to the allergens in the first place. This is impossible in some cases.
Many treatments are available to relieve symptoms, ranging from over-the-counter
antihistamines to potent anti-inflammatory drugs like steroids.
Immunotherapy, also known as desensitization
or "allergy shots" is a method of treating allergies that was developed
in 1911. In this method, extracts of the offending allergen are injected in the
individual in gradually increasing doses. Thus, the patient is allowed to build
up immunity, probably, through an increased production of protective (IgG) antibody
to combat the allergic (IgE) antibody.