Immune system is meant for giving protection against the problem causing foreign agents, but some times, over enthusiasm of immune system results in damage to its own body organs and tissue.

The violent and destructive action of immune system is referred as “hypersensitivity reaction” or “allergic reaction”.

Here the term hypersensitivity is used in a negative sense to explain the destructive action of immune system. The factors or antigens causing hypersensitivity or allergic reactions are known as allergens.

An allergen maybe a protein or a complex chemical substance with low molecular weight that generally may not cause any immunological response in non allergic people.


The allergens responsible for hypersensitive reactions may be food products or pollutants or drugs. Usually, first encounter with an allergen will not generate any hypersensitive reactions, but the immune system produces memory cells for that particular allergen.

Production of memory cells for exposed allergen is known as “sensitization”. The person with memory cells for an allergen is considered as sensitized, and shows clinical symptoms of allergy, when he encounters with the same allergen for the second time.

The hyper sensitivity reactions can be classified into two types based upon the time taken for pathogenicity.

(1) Immediate Hyper sensitivity (2) Delayed Hyper sensitivity


1. Immediate Hyper sensitivity:

If the sensitized person shows allergic reactions in less than 24 hours after exposure to an allergen, the reactions are considered as immediate hyper sensitivity reactions.

A hyper sensitivity reaction to drugs like penicillin belongs to this category. Interaction of antibodies with antigen results in immediate hypersensitivity and the reactions are handled by B lymphocytes.

Humoral immunity or antibody-mediated action is responsible for immediate hypersensitivity reactions. Therefore they are transferable from one individual to the other through serum or antibody (Ab) transfer.


2. Delayed hyper sensitivity reactions:

If a sensitized person shows hyper sensitivity reactions after 24 hours of antigenic exposure, the reactions are considered as Delayed Hyper sensitivity reactions.

Delayed hyper sensitivity reactions are cell mediated and T lymphocytes are responsible for this type of reactions. The inflammatory response can not be transferred through antibodies, but can be transferred through sensitized T cells to a non allergic person.

The clinical symptoms of delayed hyper sensitivity are Erythema (redness of skin), and Induration (hardening of tissue).


In 1963 Coombs and Gell classified hyper sensitivity reactions into 4 types, based on the mechanism of pathogenesis. They are:

(l) Type I or Anophylactic hyper sensitivity

(2) Type II or Antibody dependent cytotoxic hyper sensitivity

(3) Type III or Immune complex mediated hyper sensitivity


(4) Type IV or Cell mediated hyper sensitivity.

Apart from these 4 types, Type V or Stimulatory hyper sensitivity has been included later to Coombs and Gell nomenclature.