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allergy
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Red, patchy rash with raised areas on human skin, showing an allergic contact dermatitis reaction caused by exposure to poison ivy.
What does hypoallergenic really mean for consumer products?
This medical illustration explains a systemic anaphylactic response to bee venom in someone with type I hypersensitivity. It shows a bee sting injecting venom into the skin, which then enters the bloodstream and spreads throughout the body. The diagram highlights how the immune system reacts, including mast cell activation and the release of substances that cause symptoms such as swelling, low blood pressure, breathing difficulty, digestive issues, and skin reactions. The human figure is annotat Close-up of a patient's back showing several red, raised patches within purple dashed rectangles, indicating positive allergic reactions from a skin test.
Why do people have allergies in the spring?
Learn about hygiene hypothesis and know-how the immune system recognize and neutralize foreign bacteria from harming the body
Learn about the hygiene hypothesis and understand how exposure to certain germs help develop a child's immunity
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Red, patchy rash with raised areas on human skin, showing an allergic contact dermatitis reaction caused by exposure to poison ivy.
allergy summary
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Red, patchy rash with raised areas on human skin, showing an allergic contact dermatitis reaction caused by exposure to poison ivy.
allergy An allergic contact dermatitis reaction caused by exposure to poison ivy.

allergy

pathology
Also known as: allergenic disease, hypersensitivity
Written and fact-checked by
Britannica Editors
Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....
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allergy, hypersensitivity reaction by the body to foreign substances (antigens) that in similar amounts and circumstances are harmless within the bodies of other people. Antigens that provoke an allergic reaction are called allergens. Typical allergens include pollens, drugs, lints, bacteria, foods, and dyes or chemicals. The immune system contains several mechanisms that normally protect the body against antigens. Prominent among these are the lymphocytes, cells that are specialized to react to specific antigens. There are...

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