Source: American College of Allergy, Asthma, and Immunology
Allergy symptoms occur when your immune system overreacts to something that is harmless to most people but triggers a reaction in anyone sensitive to it. This substance is known as an allergen.
Allergies can be seasonal (worsening during certain times of the year, such as pollen allergies in the spring) or perennial (occurring year-round). Common allergens include weed or grass pollen, dust mites, animal dander, mold, insect stings and a variety of food types, such as eggs, shellfish, nuts and grains. If you come in contact with something to which you are allergic, your immune system considers it dangerous and releases a chemical called histamine to counteract it.
The release of histamine can cause a variety of symptoms, including a skin rash, headache, sneezing, runny nose, swelling, nausea and diarrhea. The most severe reaction, known as anaphylaxis, can be life-threatening.
If the allergen is something you breathe in, your reaction will most likely affect your eyes, nose and lungs. If the allergen is something you consume, you’re more likely to have symptoms in your mouth, stomach and intestines.
An appointment with a board-certified allergist can help determine the severity and cause of the problem. The allergist will discuss your medical history and symptoms and may use blood or skin tests to find out whether you have an allergy and what the allergen is. Medications (over-the-counter and prescription) and immunotherapy (allergy shots) may provide relief; making changes to your environment can also be helpful.
To learn more about specific types of allergies and their symptoms, visit the page that relates to your situation: