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Allergy

ICSE Class 7 Biology • Chapter 9 (Detailed Master Notes)

Chapter Overview

The immune system is designed to fight off dangerous invaders like bacteria and viruses. However, sometimes it gets confused. It mistakenly identifies a harmless substance—like pollen or peanuts—as a threat and launches an attack. This hypersensitive reaction by the body's immune system is what we call an Allergy.

9.1 Understanding Allergies

Allergy: An hypersensitive reaction of the immune system to specific harmless substances present in the environment.

Allergens: The specific substances that trigger an allergic reaction. Examples include dust, pollen, certain foods, or animal dander.

A person suffering from allergies is called an allergic person. A substance that is an allergen for one person might be harmless to another.

9.2 Types of Allergens

Allergens can enter the body in several ways:

Inhalant Allergens: Substances breathed in through the nose. (e.g., Pollen, dust mites, mold spores).
Ingestant Allergens (Food Allergies): Substances consumed through the mouth. (e.g., Peanuts, milk, eggs, seafood).
Injectant Allergens: Substances injected directly into the body. (e.g., Venom from bee stings or medical injections like Penicillin).
Contactants: Substances that cause a skin reaction upon direct touch. (e.g., Poison ivy, latex gloves, certain cosmetic products).

AI Image Prompt: A clean, educational grid illustration displaying common allergens. Top left: A stylized flower releasing yellow pollen grains (Inhalant). Top right: A glowing peanut and a glass of milk (Ingestant). Bottom left: A hostile-looking angry bee with a stinger (Injectant). Bottom right: A person wearing a latex glove or a metal necklace with an exclamation mark (Contactant).

9.3 The Mechanism of an Allergic Reaction

What happens inside the body during an allergy attack?

Sensitization: The first time an allergen enters the body, the immune system produces specific antibodies against it. No symptoms occur yet.
Subsequent Exposure: Next time the exact same allergen enters, those antibodies recognize it immediately.
Chemical Release: The antibodies trigger mast cells to burst, flooding the bloodstream with inflammatory chemicals, most notably Histamine.
The Symptoms: This sudden flood of Histamine causes the classic symptoms of an allergy like itching, swelling, and sneezing.

9.4 Common Symptoms

The symptoms depend on where the Histamine is released:

Nose & Eyes (Hay Fever): Caused by inhalants (pollen, dust). Characterized by sneezing, runny nose, and red, itchy eyes.
Skin (Eczema or Hives): Caused by contactants or foods. Symptoms include raised, itchy bumpy rashes.
Lungs (Asthma): Caused by inhaled allergens irritating the airways, leading to wheezing and shortness of breath.
Stomach: Caused by food allergies, leading to stomach cramps or nausea.

Anaphylaxis (MEDICAL EMERGENCY)

A highly sensitive person can experience a sudden, severe allergic reaction called Anaphylaxis. The throat may swell shut entirely, blocking breathing. This is life-threatening and requires an immediate emergency injection of a medicine called Epinephrine.

9.5 Diagnosis and Testing

To find out what is causing an allergy, doctors perform specific tests:

Skin Prick Test: Tiny drops of allergens are pricked onto the forearm. An allergic reaction creates an itchy bump.
Blood Tests: Lab tests measure allergy-specific antibodies circulating in the blood.

9.6 Management and Treatment

While most allergies cannot be fully cured, they can be managed:

Avoidance: Avoiding the known allergen completely.
Antihistamines: Medicines that block the chemical Histamine securely, quickly stopping symptoms like itching and sneezing.
Bronchodilators (Inhalers): Medicines inhaled to open up restricted swollen airways during an asthma attack.
Auto-Injectors (EpiPen): People at severe risk for Anaphylaxis carry an EpiPen. It quickly injects Epinephrine into the body to rapidly reverse life-threatening symptoms.

Practice Zone

Q1. What is an allergen?

Answer: An allergen is any harmless substance (like pollen, peanuts, or dust) that wrongfully triggers a hypersensitive, overreactive response from the immune system.

Q2. Why is histamine important in allergies?

Answer: Histamine is the chemical released by mast cells in the body when an allergen enters. It causes the symptoms of an allergy, like itching, swelling, and mucus production. This is why we take anti-histamine medicines.