What
causes allergy symptoms?
Underneath the lining of the skin, gut, lungs, nose and eyes
are mast cells. These are designed to kill worms and parasites.
Mast cells are like "land-mines", and contain "bags"
filled with irritant chemicals including histamine. When these
are released in small amounts, they cause irritation. In larger
amounts, they can cause rashes, the sneezing of hay fever and
the wheeze of asthma.
What
happens during an allergic reaction?
Mast cells are armed with proteins called IgE antibodies,
which act as remote sensors in the local environment. A person
allergic to peanut, for example, will have IgE antibodies
capable of recognising the shape of peanut protein (the allergen),
in much the same way that a lock "recognises" the shape
of a key. When this happens, mast cells dump their contents into
the tissues, causing an allergic reaction. Depending on where
the reaction occurs, it may result in itchy skin rashes, itchy
watery eyes, sneezing and runny nose, cough and wheeze or even
more serious symptoms known as anaphylaxis.
Common allergy triggers
We can have allergic reactions to what we swallow (food
or medicines), what we breathe in / inhale (eg. dust mite,
pollens, mold spores or animal allergen), what we touch
(eg. plants, animals, perfumes, cosmetic preservatives) or what
is injected (eg. insect stings, medications, blood transfusions).
Allergies tend to
run in families
Allergic people have a greater than average tendency to produce
IgE antibodies to common environmental substances. This tendency
is inherited and occurs in around 2 in 5 people. If you have one
allergic parent, for example, there is around a 1 in 3 chance
of developing allergies yourself. If you have two allergic parents,
the risk doubles. In other words, you can pick your friends but
not your parents!
Allergy is increasing
Allergy is more common now than in our parents' and grandparents'
generation. A number of careful studies in Australia and abroad
have shown that the frequency of hay fever, asthma, eczema and
even food allergy appears to have doubled in the last 30 years.
It is a true increase, and not just because doctors are better
at diagnosing allergies.
Why is allergy increasing?
There are many theories to explain the fact that the incidence
of allergy has approximately doubled in the last 20-30 years in
almost every country in which it has been studied. Theories for
which there is at least some evidence include:
The Hygiene Hypothesis
Babies are born with an immune system which is inherently biased
towards developing allergic-like reactions. These are called "type
2 response". A more balanced immune response is thought to
require early exposure to infections. The first year of life appears
to be the critical time in which this balance changes to a form
of immunity that makes allergy less likely. Early exposure to
infection is thought to "reset the balance" of the immune
system by stimulating so-called "type 1 immune responses"
instead. Recent studies have shown that multiple upper respiratory
tract infections ("colds and flu's") in the first year
of life may reduce the risk of developing asthma and hay fever
by up to 50 %. There is also evidence that exposure to some gut
bacteria may also play a role in modifying how the immune system
develops in the first year of life. If this theory is proven,
it may allow the development of new preventative strategies.
Can allergy be prevented?
This will be the subject of a separate article
References
