Sneezing at Dust Is Your Defence System Overreacting

Because your body's defence system treats it as an enemy when it is not. The reaction is real, but the threat is imagined — house dust cannot harm you, and the sneezing is entirely your own doing.

An allergy is a defence that has become oversensitive. This page covers everything in the ICSE Class 7 Biology chapter on allergy: what allergy and allergens are, common allergens and the conditions they cause, the symptoms, and how allergies are identified and treated.

An allergy is an over-sensitive reaction of the body's immune system to a substance that is normally harmless. The substance that triggers it is called an allergen.

The sequence is:

- The allergen enters the body — breathed in, eaten, injected or touched.
- The immune system wrongly identifies it as harmful and produces antibodies against it.
- On the next exposure, those antibodies cause certain cells to release a chemical called histamine.
- Histamine widens small blood vessels, makes them leak fluid, and irritates the tissues — which produces the swelling, itching, redness, sneezing and watering that you notice.

So the allergen itself does no damage at all. Every symptom of an allergy is caused by the body's own chemical response, which is why two people can share a dusty room and only one of them will sneeze.

That also explains why a first exposure often causes nothing. The body must be sensitised first — antibodies have to be made before there is anything to trigger histamine, so an allergy can appear to a food someone has eaten happily many times before.

Common allergens fall into a few groups, and each tends to cause particular conditions.

- Pollen from flowers, grasses and trees — causes hay fever (allergic rhinitis), with sneezing, a running nose and itching, watering eyes. It is worst in the flowering season.
- House dust and dust mites — cause allergic rhinitis and asthma. Mites live in bedding, mattresses, carpets and soft furnishings.
- Animal dander — flakes of skin, hair and feathers from pets and birds; causes rhinitis and asthma.
- Certain foods — commonly milk, eggs, fish, prawns, groundnuts and brinjal; cause hives (urticaria), swelling of the lips and face, and digestive upsets.
- Drugs — such as penicillin and some painkillers; cause rashes, hives and sometimes severe reactions.
- Insect stings — from bees, wasps and ants; cause local swelling and pain, and occasionally a dangerous whole-body reaction.
- Moulds and fungal spores — grow in damp walls and bathrooms; cause rhinitis and asthma.

Mould allergies are common in Indian homes during the monsoon, when damp walls encourage fungal growth.

The boundary case to mention is the severe reaction. Most allergies are uncomfortable rather than dangerous, but a sting or drug allergy can occasionally cause the throat to swell and breathing to become difficult — which needs immediate medical help, not home treatment.

The common symptoms of an allergy are:

- Sneezing repeatedly, often in bursts
- Running or blocked nose
- Watering, itching, red eyes
- Skin rashes, hives (raised itchy white or red patches) and eczema (dry, scaly, itchy skin)
- Asthma — wheezing, coughing and difficulty in breathing, because the air passages narrow
- Swelling of the lips, face or eyelids

These look much like a cold, so the differences matter:

- Cause — an allergy is caused by an allergen; an infectious disease is caused by a germ such as a virus or bacterium.
- Spread — an allergy is not contagious; an infection spreads from person to person.
- Fever — an allergy usually causes no fever; an infection often does.
- Onset — allergy symptoms begin immediately on exposure; infection symptoms develop after days of incubation.
- Pattern — allergy symptoms recur whenever the allergen is met, and stop when it is avoided.
- Itching — common in allergy, uncommon in infection.

So a student who sneezes every time a room is swept but is perfectly well otherwise has an allergy, not a cold.

The most reliable clue is the pattern. An infection runs its course and ends; an allergy returns each time the same trigger appears, which is why keeping a note of when symptoms occur is genuinely useful.

Allergies are identified by skin tests.

In a skin prick test, tiny amounts of several suspected allergens are placed on the skin of the forearm or back, and the skin is pricked lightly through each. After about fifteen minutes, any allergen the person is allergic to produces a small raised, red, itchy swelling at its own spot. The others stay normal. A patch test works similarly for substances that touch the skin, using patches left on for a longer period.

Doctors also take a careful history of when symptoms appear, and may use blood tests.

Prevention and treatment:

- Avoid the allergen — this is the most effective step of all. Keep bedding clean and sun-dried, dust with a damp cloth rather than a dry one, keep the house dry to prevent mould, and avoid a food or drug known to cause a reaction.
- Antihistamines — medicines that block the action of histamine, relieving sneezing, itching and rashes.
- Inhalers for asthma, which open the narrowed air passages.
- Creams and ointments for skin rashes.
- Desensitisation — repeated small doses of the allergen given under medical supervision, so the body gradually stops reacting.
- Wearing a mask while sweeping or cycling in dusty air.

All of these need a doctor's advice; self-medication with an unknown drug can itself cause a reaction.

The important limitation is that antihistamines treat the symptoms, not the allergy. They block histamine after it has been released, so the sneezing stops — but the sensitivity remains, and the symptoms return at the next exposure. Only avoidance and desensitisation address the cause.