Allergies are exaggerated reactions of the immune system to certain foreign invaders that it mistakes as a threat to the body. When an antigen enters the body, the immune system works to identify whether that substance is dangerous. With an allergy, the immune system incorrectly identifies a harmless substance (e.g. pollen, animal dander, eggs, milk) as dangerous.

This misidentification kicks off a series of events known as an allergic cascade that results in an allergic reaction and uncomfortable allergy symptoms (e.g. sneezing, coughing, runny nose, sore throat, diarrhea, vomiting, itching and hives). Those substances a person is sensitive to are called allergens. 

Allergens may be inhaled (e.g., pollen, dander), come in direct contact with the skin or mucous membranes (e.g., latex, poison ivy, oak and sumac), be injected (e.g., insect sting), or be ingested (e.g., peanuts, eggs).

There are three basic phases of an allergic cascade:

1. The immune system first encounters a substance that it decides is threatening and begins to produce a specific antibody called immunoglobulin E (IgE) that allows it to recognize the allergen on subsequent encounters. In other words, the person becomes sensitized to the allergen.
   
   
2. The person encounters the allergen again and IgE antibodies trigger the release of histamines and other chemicals into the bloodstream.
   
   
3. Allergy symptoms appear. They may be localized (only in the area where these chemicals were first released) or systemic (throughout the entire body). Chemicals released during an allergic reaction mainly affect the blood vessels, mucous glands and bronchial tubes. Symptoms usually involve tissue swelling and inflammation.

The most common allergies fall into one or more of the following categories:

• Indoor allergies (e.g., dust, mold, dander)
• Outdoor allergies (e.g., pollen, plants, sun, cold)
• Food allergies
• Animal allergies or insect sting allergies
• Cosmetic allergies or other chemical allergies
• Drug allergies (over-the-counter or prescription medications)

The most severe kind of allergic response an individual can have is anaphylaxis. This is a severe and potentially life-threatening reaction that occurs in certain highly-sensitive individuals. The condition is usually caused by an allergic reaction to certain foods, insect stings or drugs.

Symptoms of anaphylaxis include shortness of breath, tightness in the chest, swelling (of the lips and face), vomiting, diarrhea and lowered blood pressure. Individuals who are susceptible to anaphylaxis should carry an epinephrine injection with them, which can quickly reverse the symptoms.

Many types of allergens are triggers for people with asthma, a condition in which the airways are blocked or constricted. Allergens often cause the body to react with inflammations that further constrict airways, provoking an asthma attack in susceptible individuals. Asthma symptoms may include shortness of breath, cough and difficulty breathing.

Some people will find they are more sensitive to allergies or asthma attacks after exercising. This is because exercise – particularly in cold weather – causes the body’s temperature to rise and tiny muscles surrounding the air passageways to twitch. This restricts airflow slightly, even for several minutes after the exercise is finished.

If a person has an allergic reaction when they are already suffering from slightly impaired breathing and a higher body temperature, the results can be a much more serious event. Some people are allergic to certain foods only when they have recently exercised.