Allergy Chapter Notes | Biology Class 7 ICSE PDF Download

Introduction

• The term "allergy" was coined in 1906 by Viennese pediatrician Clemens von Pirquet, who observed that some patients exhibited hypersensitivity to typically harmless substances like dust, pollen, or certain foods. He derived the term from Greek, meaning "other work," and initially classified all forms of hypersensitivity as allergies, attributing them to improper immune system activation.
• Allergies, or allergic diseases, arise from an overactive immune response to environmental substances that are generally harmless to most people. These conditions include hay fever, dermatitis, asthma, and food allergies. 
• The substances triggering these reactions, known as allergens, vary widely and include dust, pollen, insect stings, specific foods, and more. When exposed to an allergen, the immune system produces immunoglobulin proteins that bind to the allergen and then to white blood cells, prompting the release of histamine, an inflammatory chemical. Histamine causes allergic symptoms such as inflammation, skin rashes, or bronchial constriction, as seen in asthma.
• Allergies are more prevalent in developed countries, affecting over 20% of people with allergic rhinitis (runny nose), more than 6% with at least one food allergy, and over 20% with allergic dermatitis at some point. Asthma prevalence varies by country, ranging from 1-18%. The rise in allergic diseases is linked to factors like fast food consumption, synthetic materials, and environmental pollutants.

Entry Routes of Allergens

• Skin: Contact with substances like latex, certain fabrics, or insect bites often triggers allergic reactions on the skin.
• Mouth: A wide range of foods can cause allergic reactions, with 90% of food allergy responses attributed to cow's milk, soy, eggs, wheat, peanuts, fish, and shellfish. Less common food allergies, affecting fewer than 1 in 10,000 people, are considered rare. Allergy prevalence varies between children and adults, with milk-protein allergies being more common in children. Some individuals cannot tolerate milk from cows, goats, or sheep, nor dairy products like cheese. Approximately 10% of children with a milk allergy may also react to beef, which contains small amounts of a protein found in cow's milk.
• Nose: Inhaled allergens such as pollen, dust, or chemicals can enter through the nose, leading to allergic symptoms.

Signs and Symptoms of Allergies

• Airborne allergens (dust, pollen) affect eyes, nose, and lungs.
• Symptoms include nasal irritation, sneezing, coughing, wheezing, red and watery eyes, increased mucus, and shortness of breath.
• Food allergies cause abdominal pain, bloating, vomiting, diarrhea, itchy skin, and swelling.
• Food allergies rarely cause asthmatic reactions.
• Insect bites, antibiotics (e.g., penicillin), and medicines (e.g., aspirin) may cause anaphylaxis, a severe systemic reaction.
• Anaphylaxis symptoms include skin redness (over 10 cm), rashes, swelling, bronchoconstriction, and low blood pressure.
• Anaphylaxis can start suddenly or be delayed and may recur after seeming to subside.

Types of Allergies and Their Symptoms

Food Allergy:

• Milk: Causes vomiting and hives.
• Wheat: Leads to stomach upset, eczema, and asthma-like symptoms (bronchospasms).
• Seafood: Results in vomiting and, in severe cases, anaphylaxis.
• Nuts: Causes vomiting, red rashes, and itching.

Seasonal Allergy:

• Spring: Pollen from blossoming flowers causes sneezing and sniffling.
• Summer: Pollen sensitivity may persist, causing similar symptoms.
• Winter: Pollutants like sulfur dioxide and nitrogen dioxide in smog cause coughing, itchy eyes, sneezing, runny nose, and watery eyes.

Dust Allergy: Dust particles trigger asthma.

Drug Allergy: Penicillin causes diarrhea, nausea, and hives.

Common Allergic Reactions

• Hay Fever: Caused by pollen, common during flowering seasons, leading to inflamed nasal lining, runny nose, sneezing, itchy, sore, and watery eyes.
• Asthma: Bronchiole muscles contract, narrowing airways, causing wheezing and breathing difficulty, triggered by pollen, dust, or certain foods.
• Hives: Swollen, pale red bumps or patches on skin due to allergens like medicines or food.
• Anaphylaxis: Sudden, severe reaction causing low blood pressure and shortness of breath, which may recur over time.

Precautions to Avoid Allergic Reactions

• No permanent cure exists for allergies, but medications provide relief.
• People prone to allergies should identify and avoid specific allergens, especially foods and medicines.
• Anti-allergy shots help with seasonal allergies like pollen reactions.
• Consult allergy specialists for severe cases.

Diagnosis of Allergy

• Accurate diagnosis is key to managing allergies effectively.
• Allergy testing confirms or rules out allergies, reducing symptoms and medication needs.
• Two methods: Skin prick test and allergy blood test, both equally valid.

Skin Prick Test

• Also called puncture or prick testing, involves tiny pricks on the skin.
• Uses a small device to prick or inject allergens (e.g., pollen, insect sting proteins, peanut extracts) into the skin, usually on the forearm or back.
• A drop of allergen fluid is placed on the skin, and the skin is pricked through it.
• If allergic, a reaction (redness or hive) occurs within 30 minutes.
• Control drop (usually saline) is included to compare reactions and confirm allergen-specific responses.

Blood Testing

• An allergy blood test is a simple, quick procedure that can be ordered by a licensed healthcare provider, such as an allergist. Unlike skin prick tests, it is suitable for all ages, skin conditions, medications, symptoms, disease activity, and during pregnancy. 
• The test is conducted by sending a blood sample to a laboratory, where it is analyzed for multiple allergens using a single sample. It is highly safe, as it involves no direct exposure to allergens. 
• The test measures levels of specific antibodies in the blood, helping identify allergens, monitor disease progression, assess the risk of severe reactions, and explain cross-reactivity between allergens.