Allergies and Hypersensitivities - Definitions
Allergy
An allergy is an unexpected reaction that follows the second or subsequent exposure to a particular substance. This substance, called an allergen or antigen, may be a food, bacteria or a pollen. Indeed almost any environmental substance can act as an allergen such as moulds, food preservatives, dyes, drugs and vaccines. Substances can enter the body by being breathed in, swallowed, touched or injected.
Like people, some dogs appear to be particularly sensitive to some of these substances.
Hypersensitivity
Following the primary exposure to the allergen subsequent exposure to the same substance can result in hypersensitivity. This allergic reaction can be described as immediate or delayed and either systemic (involving the whole body) or local.
Anaphylaxis (please see separate handout) is an example of an immediate type of hypersensitivity. It can also be systemic in that the whole dog is involved.
Contact dermatitis, on the other hand, is an example of a local hypersensitivity since it only affects the skin that has been in contact with the allergen, e.g. the face, abdomen, paws etc. To complicate matters contact dermatitis may be immediate, i.e. itchiness, swelling or a rash developing as soon as the dog had come in contact with the allergen, e.g. grass pollen, or it may be delayed and only occur some time after contact.
© Copyright 2025 LifeLearn Inc. Used and/or modified with permission under license. This content written by LifeLearn Animal Health (LifeLearn Inc.) is licensed to this practice for the personal use of our clients. Any copying, printing or further distribution is prohibited without the express written consent of LifeLearn. This content does not contain all available information for any referenced medications and has not been reviewed by the FDA Center for Veterinary Medicine, or Health Canada Veterinary Drugs Directorate. This content may help answer commonly asked questions, but is not a substitute for medical advice, or a proper consultation and/or clinical examination of your pet by a veterinarian. Please contact your veterinarian if you have any questions or concerns about your pet’s health. Last updated on Nov 3, 2016.
