When we mention the word “Allergy”, majority of people correlate it with “Aachoo” – sneezing, running nose, watering of eyes, cough, etc. Some people think of allergy to medicines like Penicillin, Sulfa drugs, etc. and others connect it to food substances like milk, meat, cheese, etc.
Unfortunately, allergy has become the bane of modern civilisation and we find ourselves allergic to several substances like pets, insects, perfumes, wheat, food preservatives, latex (gloves), plants, metals like arsenic, etc. Allergy is also a complicated and enigmatic subject for doctors and scientists, since neither the causative mechanism is clear, nor the treatment is satisfactory.
In order to understand what allergy is all about, we must first understand its basic mechanism. Allergy is closely related to our Immune System – the system which protects us from sickness and diseases. Our body is so designed that whenever a foreign invader (bacteria, virus, chemical, heat, cold, wind, water, etc.) or inner enemy (tumour, aged cells, etc.) attacks our body, the immune system gets activated to protect us from this invader.
The innate (inborn) immune system consists of barriers such as skin and mucous membranes, enzymes and white blood cells (WBC) called neutrophils in its armamentarium. If the innate immune system is unsuccessful, the adaptive immune system takes over.
This system consists of cells such as lymphocytes and monocytes and their products such as antibodies, cytokines and antigen-specific cytotoxic (killer) cells. This system does not damage the body tissues, but destroys the abnormal material (invader). It also remembers the encounters with specific invaders such that subsequent encounters by the host with the same agents activate the immune response more quickly and vigorously, preventing recurrence of the disease caused by the invader.
In contrast to the adaptive immune system, in certain individuals there is an over-reactive immune system, i.e. there is an abnormal reaction to the ordinarily harmless substances (invaders). This disorder is called an Allergy or Hypersensitivity.
What Happens During an Allergic Reaction
When a person with a hypersensitive or hyper-alert immune system is exposed to an invader (an allergen), a series of events take place :-
1. The WBC of the body start to produce a specific type of antibodies called Immunoglobulin E (IgE) to fight the allergen.
2. These antibodies attach to another types of cells called mast cells, which are found in the airways and in the digestive system where the allergens enter the body.
3. The mast cells release a variety of chemicals like histamine, serotonin, etc. which produce localised effects like sneezing, running nose, watery eyes, itching, diarrhoea, vomiting, etc. or generalised effects like breathing difficulty, low blood pressure, swelling of body, weakness, loss of consciousness, shock or even death.
There are two major differences between the normal immune reaction and allergic reaction:-
1. In a normal individual there is an antigen-antibody reaction, which protects the individual (host) while in a person with allergy, the reaction of allergen with tissue-bound antibody (IgE) can lead to several unpleasant symptoms to the body and causation of disease.
2. In an allergic person, the body does not produce antibodies (IgE) unless he has been sufficiently exposed to the allergen earlier and for a prolonged period. Hence in cases of Food Allergy, the person who has been eating the (allergic) food substance regularly is more prone to develop a reaction than to a new substance recently added to his diet.
There is another type of allergic reaction seen in allergic individuals, which is known as Delayed Hypersensitivity Reaction or Cellular Allergy. In this case, the body, instead of producing IgE antibodies, has overreactive lymphocytes produced by thymus gland. When exposed to certain allergic substances which come into direct contact with the skin, this type of allergic reaction is observed. The lymphocytes while reacting to get rid of the foreign material or toxin give rise to certain skin changes causing a disease called Contact Dermatitis.This type of allergic reaction is also seen when the body rejects the organs transplanted in the body