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An allergen is a normally harmless substance, such as pollen or peanuts, that the immune system mistakenly treats as a dangerous invader. Free notes.
Immune System Problems and Allergic Reactions is part of Animal tissues, organs and organ systems in GCSE Edexcel Higher Biology.
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An allergen is a normally harmless substance, such as pollen or peanuts, that the immune system mistakenly treats as a dangerous invader. Free notes.
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Use this lesson to stay inside Animal tissues, organs and organ systems while connecting the detail back to the wider Topic 5: Health, disease and the development of medicines topic.
Read the core explanations from Immune System Problems and Allergic Reactions before testing yourself from memory.
Your immune system is brilliant at fighting off dangerous pathogens like bacteria and viruses. But sometimes it makes a mistake - treating harmless substances like deadly invaders. This overreaction is what we call an allergic reaction.
Picture this: your immune system encounters pollen for the first time. Instead of ignoring it (which would be sensible), it panics and treats it like a dangerous pathogen. It creates antibodies and prepares for war. The next time you breathe in pollen, your immune system launches a full attack - causing sneezing, runny nose, and watery eyes.
This explains why allergies often develop after repeated exposure. Your immune system needs time to 'learn' to overreact to the allergen.
When your immune system first encounters an allergen, it goes through a process called sensitisation. Think of this as your immune system creating a 'wanted poster' for the allergen. It produces specific antibodies (called IgE antibodies) that will recognise this substance in future.
These antibodies attach to special immune cells called mast cells, which are loaded with chemical weapons - particularly histamine. When the allergen appears again, it binds to the antibodies, triggering the mast cells to release their contents.
Histamine is the main culprit behind most allergy symptoms. It causes blood vessels to widen (making you red and swollen), increases mucus production (runny nose), and makes you itch. In severe cases, it can cause airways to narrow, making breathing difficult.
The AQA specification focuses on two main types of allergic reactions: skin rashes and asthma. Both are caused by the same underlying immune overreaction, but they affect different parts of your body.
Skin rashes (eczema, hives): When allergens contact your skin or enter your bloodstream, mast cells in your skin release histamine. This causes the classic symptoms - redness, swelling, itching, and sometimes blisters. Common triggers include certain foods, soaps, metals like nickel, or even stress.
Asthma: This affects your airways - the tubes that carry air to your lungs. When allergens are inhaled, mast cells in your airways release histamine and other chemicals. This causes three problems: the airway walls swell, extra mucus is produced, and the smooth muscles around the airways contract. The result? Wheezing, coughing, and difficulty breathing.
Trap: Many students think asthma is just about 'weak lungs' or pollution. Actually, people with asthma often have perfectly healthy lungs - it's their immune system that's overreacting to specific triggers.
Here's the key insight for your exams: allergic reactions use the same immune mechanisms that normally protect you from pathogens. The problem is that these powerful weapons are being fired at the wrong targets.
When your immune system fights a real pathogen like a virus, it produces antibodies, activates immune cells, and releases chemicals to destroy the invader. This inflammation and immune response helps you recover. But when the same response targets harmless pollen or peanuts, you get allergy symptoms instead of protection.
Worked example: How Immune Reactions Lead to Allergies
Explain the sequence of events from first allergen exposure to allergic reaction
Answer: The immune system's normal pathogen-fighting mechanisms are misdirected against harmless substances, causing allergic symptoms instead of protection
Allergy questions often appear in the context of immune system topics. Examiners love to test whether you understand that allergies are immune overreactions, not immune deficiencies.
Command word focus: 'Explain' questions about allergies want you to describe the cause-and-effect relationship. Start with the immune system mistaking a harmless substance for a pathogen, then follow through to histamine release and symptoms.
Common mistake: Don't say people with allergies have 'weak immune systems' - it's the opposite. Their immune systems are overactive against specific substances.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-23.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
The symptoms of asthma are wheezing, coughing and difficulty breathing. They happen because the airway walls swell, extra mucus is produced and the smooth muscles around the airways contract.
The symptoms of a pollen allergy are sneezing, a runny nose and watery eyes. They happen when the immune system, after treating pollen as a dangerous pathogen, launches a full attack the next time pollen is breathed in.
Mast cells are special immune cells loaded with chemicals, particularly histamine. IgE antibodies attach to mast cells, and when the allergen binds to these antibodies, the mast cells release histamine, causing allergy symptoms.
An allergen is a normally harmless substance, such as pollen or peanuts, that the immune system mistakenly treats as a dangerous invader. Repeated exposure can trigger an allergic reaction.
Histamine is a chemical released by mast cells during allergic reactions that causes inflammation, swelling and other allergy symptoms. It makes blood vessels widen, increases mucus production and causes itching, and in severe cases it can make airways narrow, making breathing difficult.
In asthma, inhaled allergens make mast cells in the airways release histamine and other chemicals. The airway walls swell, extra mucus is produced and the smooth muscles around the airways contract, causing wheezing, coughing and difficulty breathing.
An allergy develops through sensitisation: on first exposure, the immune system mistakes the allergen for a threat and produces specific IgE antibodies, which attach to mast cells. On re-exposure, the allergen binds to these antibodies, triggering the mast cells to release histamine.
IgE antibodies are specific antibodies produced during sensitisation that recognise an allergen. They attach to mast cells, and when the allergen appears again and binds to them, the mast cells release histamine.
An allergic reaction is an overreaction of the immune system to a normally harmless substance, such as pollen or peanuts, causing symptoms like rashes or breathing difficulties. The immune system uses the same mechanisms that normally protect against pathogens, but fires them at the wrong target.
People with allergies do not have weak immune systems; it is the opposite. Their immune systems are overactive against specific substances, so allergies are immune overreactions, not immune deficiencies.
Sensitisation is the first encounter of the immune system with an allergen, when it mistakenly identifies the harmless substance as a threat. It produces specific IgE antibodies, like a wanted poster, that will recognise the allergen in future.
Allergic skin rashes, such as eczema and hives, are caused when allergens contact the skin or enter the bloodstream and mast cells in the skin release histamine. Common triggers include certain foods, soaps, metals like nickel, or even stress.
Asthma is not caused by weak lungs or pollution. People with asthma often have perfectly healthy lungs; their immune system is overreacting to specific triggers.