The word ‘allergy’ means abnormal tissue reactivity after the body is exposed to a foreign substance. You must have seen that some persons develop a rash if they eat certain food such as fish, or start sneezing when exposed to dust, cat-fur or pollen. Such persons are hypersensitive or allergic to these substances.

The unfavourable reaction of the body to these substances, which produce no obvious ill effects in others, is called an allergic reaction. An allergic reaction may occur when there is a contact between a foreign protein and body tissues, which are sensitive to it.

Allergic reactions may be trivial and local such as a mild urticaria, or may be general and so serious as to result in sudden death. Serious allergic reactions are known as anaphylactic shock. Allergic reactions may affect any stream of the body producing different symptoms. Asthma, hay fever, migraine, urticaria, some forms of eczema, swelling of joints, rashes are some examples of allergic reactions.

Most allergic can be prevented by simply avoiding the substances to which the body is sensitive. The drugs called antihistamines are effective in controlling mild and local forms of allergy. For the treatment of allergic asthma and anaphylactic shock, adrenaline and its derivatives are quite useful.


Immunity is the ability of an individual to rest the development of disease. The natural defence mechanism of the body to resist infections is called immune system.

Allergic disease occurs due to malfunctioning of immune system of the affected individual. The immune system of an individual may become weak due to malnutrition, inherited metabolic disorders, exposure to radiation or pathogenic (disease causing) micro-organisms. Due to weaken immunity, the body becomes susceptible to attack by various diseases during causing micro-organisms.

For examples the disease AIDS (Acquired Immune Deficiency Syndrome) which is caused by a virus, weakens the immune system of the patient. As a result, a patient suffering from AIDS becomes susceptible to all diseases and soon dies due to various infections that attack him.