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.


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 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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