ALLERGY AND IMMUNOLOGICAL HYPERSENSITIVITY REACTIONS
The term allergy means
hypersensitivity. It is defined as abnormal immune response to a chemical or
physical agent (allergen).
During the first exposure to an allergen, the immune response does not
normally produce any reaction in the body. Sensitization or an initial exposure
to the allergen is required for the reaction. So, the subsequent exposure to
the allergen causes variety of inflammatory responses. These responses are
called allergic reactions or immunological hypersensitivity reactions.
Immunological hypersensitivity
reactions may be innate or acquired. These reactions are mediated mostly
by antibodies. In some
conditions, T cells are involved. Common symptoms include sneezing, itching and
skin rashes. However, in some persons the symptoms may be severe.
Common allergic
conditions are:
1. Food allergy
2. Allergic rhinitis
3. Bronchial asthma
4. Urticaria.
ALLERGENS
Any substance that
produces the manifestations of allergy is called an allergen. It may be an
antigen or a
protein or any other
type of substance. Even physical agents can develop allergy.
Allergens are
introduced by:
1. Contact (e.g.:
chemical substance)
2. Inhalation (e.g.:
pollen)
3. Ingestion (e.g.:
food)
4. Injection (e.g.:
drug).
Common Allergens
1. Food substances: Wheat,
egg, milk and chocolate.
2. Inhalants: Pollen
grains, fungi, dust, smoke, perfumes and disagreeable odor.
3. Contactants: Chemical
substances, metals, animals and plants.
4. Infectious agents: Parasites,
bacteria, viruses and fungi.
5. Drugs: Aspirin
and antibiotics.
6. Physical agents: Cold,
heat, light, pressure and radiation.
IMMUNOLOGICAL
HYPERSENSITIVE REACTIONS
Immunological hypersensitive
reactions to an agent give rise to several allergic conditions and autoimmune diseases.
Hypersensitive
reactions are classified into five types:
Type I or anaphylactic
reactions.
Type II or cytotoxic
reactions.
Type III or
antibody-mediated reactions.
Type IV or
cell-mediated reactions.
Type V or
stimulatory/blocking reactions.
Type I or
Anaphylactic Reactions
Anaphylaxis means
exaggerated reactions of the body to an antigen or other agents to which the
body is sensitized already. It is also called immediate hypersensitive reaction
because it develops within few minutes of exposure to an allergen. Anaphylactic
reactions are mediated by IgE and other factors involved in inflammation
(inflammation means the protective response of the tissues to the
damage or destruction
of cells). When the body is exposed to an allergen, the IgE immunoglobulins are
produced. Also called reagins or sensitizing antibodies, these immunoglobulins
bind with the surface receptors of mast cells and circulating basophils. Mast
cells are the granulated wandering cells
found in connective
tissue and beneath the mucous membrane in the throat, lungs and eyes. During
subsequent exposure of the body to the same allergen, the allergen IgE antibody
reaction takes
place. This leads to degranulation
of mast cells and basophils, with the release of some chemical mediators such
histamine. The chemical mediators produce the hypersensitivity reactions. Most
serious reactions are fall in blood pressure (due to
vasodilatation),obstruction of air passage and difficulty in breathing (due to bronchoconstriction)
and shock.
Type II or Cytotoxic
Reactions
Cytotoxic reactions
involve mainly the IgG antibodies, which bind with antigens on the surface of
the cells,
particularly the blood
cells. The affected cells are destroyed. Sometimes, IgM and IgA antibodies are
also involved. The
diseases developed due to cytotoxic reactions are hemolytic diseases of newborn
in case of Rh incompatibility and autoimmune hemolytic anemia.
Type III or
Antibody-mediated Reactions
Excess amounts of
antibodies like IgG or IgM are produced in this type. The antigen-antibody
complexes
are precipitated and
deposited in localized areas like joints causing arthritis, heart
causing myocarditis
and glomeruli of kidney producing glomerulonephritis.
Type IV or
Cell-mediated Reactions
This type of
hypersensitivity is also called delayed or slow type of hypersensitivity. It is
found in allergic reactions due to the bacteria, viruses and fungi. It is also
seen in contact dermatitis caused by chemical allergens and during rejection of
transplanted tissues. An example of type IV reaction is the delayed reaction
after intradermal injection of tuberculin in persons who are previously affected
by tuberculosis (tuberculosis skin test or Mantoux test).
The important feature of delayed type of hypersensitivity is the involvement of
T lymphocytes rather than the antibodies.
Type V or
Stimulatory/Blocking Reactions
It is seen in
autoimmune diseases like Graves’ disease (stimulatory reactions) and myasthenia
gravis (blocking reactions).
Graves’ disease: Normally,
TSH combines with surface receptors of thyroid cells and causes synthesis and secretion
of thyroid hormones. The secretion of thyroid hormones can be increased by
thyroid-stimulating antibodies (TSAB) produced by plasma cells (B lymphocytes).
The excess secretion of thyroid hormone leads to Graves’ disease. Myasthenia gravis: It is
due to the development of IgG autoantibodies.
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