An allergic reaction is defined as an excessive reaction by the
body when exposed to substances which usually do not induce any
specific reaction. This allergic reaction assumes that there was
prior sensitization.
Such sensitization occurs during activation of T lymphocytes by
an allergen. This activation can involve two types of T cells: Th1
lymphocytes and Th2 lymphocytes. Th2 lymphocytes cause production
of IgE subsequent to exposure to a minute quantity of an allergen
(by inhalation, ingestion, injection or more rarely by contact) and
is the source of a cascade of immune reactions responsible for
symptoms experienced by the patient.
In the so-called delayed hypersensitivity (delayedcell-mediated
hypersensitivity), the allergen causes stimulation of Th1
lymphocytes which are responsible for the symptoms observed in the
patient. This mechanism is involved in contact dermatitis and in
some drug-related allergic reactions.
Many substances can cause an allergic reaction (see
section Allergens).
Similarly, a wide number of allergic disorders exist (see section
Allergic Diseases).
Coombs and Gell developed a classification system of allergic
reactions into 4 basic types. During the course of advances in
knowledge, this classification has often been called into question.
However, it continues to be a common thread for advancing along the
pathway of allergy. It is also necessary to note the frequent
overlap of these 4 types of allergy, whose interest is mainly
educational. We can differentiate the following:
Type I reaction
So-called “anaphylactic reaction”, even though this term may
cause confusion.
IgE antibodies bind to mast cells in tissues or basophils in the
blood. When binding to the IgE-cell complex, the antigen triggers
the release of powerful mediators by the cells (degranulation).
Those mediators (histamine, serotonin, heparin, various enzymes)
are the cause of the allergic reaction which occurs rapidly and
thus a type I reaction is called an "immediate-onset allergy".
A type I reaction is involved in the following allergic
disorders:
- Allergic asthma
- Allergic rhinitis
- Acute urticaria
Type II reaction
So-called cytotoxic reaction. In this case IgG or IgM reacts
with the antigen when it is bound to the cell wall of the target
cell (mainly a blood cell). This antigen can also be
a <HAPTEN>. This binding is not deleterious but may
activate a macrophage. When the complement is also activated, then
reactions are enhanced, progressing to cytolysis.
The mechanism for type II is involved in the following
disorders:
- A drug-related agranulocytosis
- Purpura by drug-related hypersensitivity
- Certain drug-related hemolytic anemias
Type III reaction
Circulating antigen-antibody immune complexes are deposited in
walls of blood essels or renal glomeruli, and then bind to the
complement. When this reaction is localized, a violent reaction,
the so-called “Arthus phenomenon” occurs together with thrombosis
and bleeeding. If such a reaction is systemic, this process is
called "serum sickness". Generally, this reaction occurs a few
hours after contact with the allergen, and hence the name
“semi-delayed type allergy”.
The type III mechanism is the cause of the following
disorders:
- Serum sickness
- Connective tissue disease including lupus erythematosus
- Allergic vascularitis
- Certain urticaria
- Glomerulonephritis caused by immune complexes
- Allergy to penicilin, phenacetin, rifampicin
Type IV reactions
So-called cell-mediated reactions: the cause is not a
circulating antibody, but a sensitized lymphocyte which releases
soluble mediators (interleukins) after a contact with the antigen.
This reaction occurs whithin 24-48 hours and hence the name
“delayed-onset allergy” is given. The most widely-known example of
this mechanism is the tuberculin test.
The type IV mechanism is the cause of the following
disorders:
- Contact dermatitis or eczema
- Graft rejection
- Autoimmune diseases
- Certain drug-related allergies
