Allergy
In Japan patients often say, “I
have an allergic diathesis”, or “This disease is caused by an allergy.” The
term ‘allergy’ should be taken to mean the specifically altered state of a host
following exposure to an allergen. This biological state or allergy has itself
either clinical hypersensitivity or clinical immunity. In this report I would
like to discuss hypersensitivity. In the case of clinical hypersensitivity,
when an antigen enters the body, a circulating antibody will be produced. If
the same antigen enters again, the antibody will react to this antigen, and
chemical mediators, e.g. histamine, will be released from mast cells. Chemical
mediators bring about allergic disorders, such as urticaria or pollinosis (hay
fever).
Coombs and Gell classified
allergic reactions into four types (I to IV). A Type I reaction is called an
immediate type, where an antigen antibody reaction occurs within some tens of
minutes. Urticaria, pollinosis (hay fever), asthma, anaphylaxis, and atopy are
caused by Type I reactions. Extensive urticaria may be an expression of
cutaneous anaphylaxis. Asthma and pollinosis (hay fever) are manifestations of
allergic reactions caused by inhaled antigens. The term ‘atopy’ was introduced
by Coca in 1929 as a convenient collective term for a group of diseases. Atopy
is a genetically determined disorder in which there is an increased tendency to
form reagin (IgE) antibodies. Atopic dermatitis and some disorders resulting
from insect stings and food allergies are associated with reagin antibodies,
and therefore are grouped with the atopic diseases. Some allergic disorders
such as pollinosis (hay fever), urticaria, and asthma can occur in any
individual who under special circumstances produces reagin antibodies, and who
has a familial predisposition to do so. Atopic diathesis is hereditary and it
is said that if parents are susceptible to pollinosis or urticaria, their
children are likely to suffer from atopic dermatitis or asthma. In the case
where an infant simultaneously suffers from atopic dermatitis and asthma, there
is a possibility that the asthma will get better as the atopic dermatitis gets
worse, and vise versa.
Atopic dermatitis and contact dermatitis, considered to be
the most common among the various types of skin diseases, are representative of
diseases that result from Type IV reactions. A Type IV reaction is called a
delayed type in which an antibody (sensitized lymphocyte) reacts to an antigen
after 24-48 hours. Here I would like to emphasize that the dermal symptoms of
atopic dermatitis are caused by a Type IV allergic reaction, and that the
patient, at the same time, has atopic diathesis with increased production of
IgE antibodies which can be attributed to a Type I allergic reaction. The
patient with atopic diathesis may possibly react to various stimuli and have
strange allergic phenomenon (see next page). This is the reason why atopy is
called ‘a strange disease.’
Most allergodermia such as contact
or photosensitive dermatitis are due to an antigen antibody reaction with a
certain factor, which, having once entered the body later enters it again. This
means that these allergic reactions occur as the result of hypersensitivity to
a certain substance, not because of an allergic or atopic diathesis.
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