Symptoms
The most common symptom associated with allergies is itching of the skin, either localized (one area) or generalized (all over the body). Another group of symptoms involves the respiratory system with coughing , sneezing, and/or wheezing. Sometimes, there may be runny discharge from eyes or nose. The third manifestation involves the digestive system, and the dog may vomit or have diarrhea.

What is flea allergy and how can it be treated?
Flea allergy is the exaggerated inflammatory response to a flea bite. Flea saliva is the allergen. It is a common allergy of dogs, although only a minority of dogs become allergic. Most dogs experience minor irritation from flea bites. But the flea allergic dog will react to a single bite with severe local itching. It will bite and scratch itself and may remove large amounts of hair. Secondary bacterial infection may occur in the broken skin. The area most commonly involved is over the rump in the tail base region.
Because one flea can be a problem for the allergic dog, strict flea control is essential. This is difficult considering the life-cycle of fleas, but there are means for instituting an intensive flea elimination program in the house. Your veterinarian can give you tips on protecting your dog from fleas.

What is a food allergy? 
Food allergy is one of the five most common allergies or hypersensitivity known to affect dogs. Most people know someone who is allergic to certain foods, such as strawberries or nuts.  It is only recently that food allergies have become recognized in dogs.  The signs are usually itchy skin or an upset stomach.  Other more subtle changes can also occur including hyperactivity, weight loss, lack of energy and even aggression.

What are signs of food allergy?
Many dogs will occasionally react to something they ate. This may be sensitivity to a particular type of food. The symptoms are often not a true allergy, just mild gastrointestinal upset. Once you associate the upset with a particular food and avoid it, the problem is usually solved.
Food allergy is different. Antibodies are produced against some part of the food, usually a protein. In a pet with food allergy, the immune system overreacts and produced antibodies to substances that it should normally tolerate. This excessive response is termed an allergic reaction. Most pets with food allergies have itching rather than vomiting or diarrhea.
Food allergies can show up as early as five months, and as late  as twelve years old.  Majority of the time food allergies show up between the ages of two and six. When finding out that your pet has a food allergy, you usually are told that you need to change your pets diet.  Sometimes a hypoallergenic diet will be prescribed.  Putting your dog on a new diet will take about 8-12 weeks or more to make sure that the diet is working. The most common food culprits for dogs are:
Beef
Dairy Products
Chicken
Wheat
Chicken Eggs
Corn
Soy

What is contact allergy?

Contact allergy is the least common type of allergy in dogs.  It results from direct contact to allergens contained in flea collars or bedding, such as pyrethrins or wool.  If the dog is allergic to these substances, there will be skin irritation and itching at the points of contact. Removal of the allergen (once it can be identified) solves the problem.

Inhalant allergies

What causes it? 
There are a wide variety of allergens that can cause atopy.  These  are similar to the causes of hay fever or human asthma.  When the affected individual inhales dust, pollens or molds, the allergic response occurs. 

What happens to the dog when this occurs?
Atopy in the dog is usually characterized by seasonal, generalized itching. Your dog chews, licks,and scratches almost the entire body, especially the feet and face.  Saliva will often stain light colored hairs resulting in orange or reddish brown hair. The dog may also scratch and rub the face, particularly around the eyes and ears.  The axillae (armpits), groin and the inside of the thighs may also be affected.
In humans, inhaled allergens cause respiratory problem, hay fever or asthma. Dogs will occasionally have respiratory signs, in addition to the pruritis (itching).

How do you find the cause of my dog's inhalant  allergy?
Diagnosis is not easy. It is based on the presence of clinical signs and ruling out other causes of pruritis such as flea allergy dermatitis or food allergies. The itching caused  by grass pollen is the same as house dust mites and many molds.  In other words, your dog may be allergic to several different things with the end result being the same, excessive itching and scratching.
A thorough medical history will help narrow the causes.  For example, if the itching occurs in the spring when certain pollen is prevalent, this narrows the field of investigation.

I understand my dog will have allergy tests to make a diagnosis. Is this true?
Approximately 80% of allergy diagnoses can be confirmed by allergy testing.  There are two primary methods of allergy testing. The tradional allergy test is called intra-dermal allergy testing and consists of injecting a tiny amount of allergen into the skin. If the body produces a response to the allergen, the body is said to be allergic to that substance. The newer allergy test involves taking a blood sample and is called IgE allergy testing.  The blood is evaluated for the presence of immune cells against certain allergens. If the body contains a high number of these IgE antibodies, an allergy to that allergen exists. We can discuss with you the best type of testing for your pet's condition.
Once the diagnosis has been made, it is possible to desensitize the dog. This involves the use of specific antigen injections that can be created according to the results of the allergy tests.  The theory is that the controlled injections of increasing amounts of the offending allergens "reprogram" the dog's immune system to reduce its response. For most dogs, allergen therapy of "allergy injections" result in significantly reduced itching and may be completely curative in some.

If this does not work, what else can be done?
Anti- flammatory drugs including corticosteroids and antihistamines will often bring relief of itching. In addition, the use of certain omega fatty acids help most pets with allergic skin disease. These approaches treat only the clincial signs, not the underlying allergy.

Bacterial Allergies
This is the least common allergy in an animal.  Many types of Staphylococcus bacteria can cause allergies.  When a dog gets bacteria like this, you will notice hair loss that looks like ringworm.  It is usually round and 1/2 to 2 inches in diameter. To help your animal with this kind of bacteria, it will have to be put on antibiotics.