Your dog’s skin during an allergic reaction

by Audrey Harvey

Have you ever wondered what is actually happening in your dog’s skin during an allergic reaction? What causes the redness, and why does he itch? You’re just about to find out!

Firstly, let’s define the term “allergen”. An allergen is anything that causes an allergic reaction. It could be dust, it could be pollen, or it could be the saliva injected into your dog’s skin when a flea bites. They are usually very small protein molecules, and although most are absorbed through the skin, they can also be absorbed through the nose or intestinal wall.

We also need to understand a little about mast cells. These cells are part of your dog’s defense system and as such, are found in tissues that interact with the outside world such as his skin, his digestive tract and his nose.  They are programmed to kill parasites rather than bacteria or viruses, and contain small granules of histamine and other chemicals which cause inflammation. These granules are released in response to infection with a parasite, in the hope that the chemicals destroy the invader.

When your dog is exposed to a potential allergen for the very first time, his immune cells produce antibodies against it. These antibodies then attach themselves to mast cells in his skin. At this stage, there’s not a problem, and your dog appears just fine.

The problem occurs the next time he comes across this allergen. It binds to the antibodies that are attached to the mast cells, and causes the cells to release their little bundles of chemicals. These chemicals cause that familiar itching, swelling and reddening of his skin.

Managing your dog’s allergies involves preventing his immune system being exposed to allergens, as well as reducing his body’s reaction to them. 

Because many allergens are absorbed through the skin, careful washing with an appropriate shampoo is a very important part of caring for an allergic dog. A mild shampoo such as Comfy Dog will gently cleanse your dog’s skin and help to remove allergens, without drying his coat. A liberal application of Fur Butter after his bath will condition his coat and leave it soft and smelling great. Both products contain colloidal oatmeal which is known to reduce skin inflammation and itching. No-one has been able to identify exactly how it does this, but its effects have been proven.

Most people have heard of antihistamines, and are familiar with their use to treat symptoms of allergies. These drugs can’t stop the release of histamine from mast cells, but they can block its effects on your dog’s skin. By doing this they help to ease inflammation and give your dog relief from itching. The other common medication used to treat allergies is corticosteroids. They work by suppressing your dog’s immune system.  Antihistamines aren’t always effective in dogs, and corticosteroids can have some unwanted side effects.

Desensitizing injections can allow your dog’s immune system to learn to tolerate allergens, so there is less chance of histamine release in his skin. This treatment is effective in up to 80% of dogs, but it’s very costly.

If you treat your dog’s allergies from all angles, you’ll get the best possible results with the least chance of side effects.


Audrey Harvey is a veterinarian who has worked in small animal practice for 20 years, and has been involved in teaching and competing in dog obedience and agility. She is passionate about preventative health care in dogs, particularly obesity management and the prevention of boredom related behavioral problems. Audrey lives in Brisbane Australia, and shares her couch with an Australian Cattle Dog, an Australian Working Kelpie and two Whippets.

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