Allergies represent an overreaction of your body's immune system to an irritant such as pollen, dust mites, cat and dog dander, mold and stinging insects. Exposure to an allergen sets off a reaction that produces histamines, chemicals that cause intense reactions in an attempt to rid your body of the allergen, such as coughing, sneezing, runny nose and watery eyes. If allergic reactions are severe enough to disrupt daily life, allergy shots, called immunotherapy, are given to desensitize you to the substance. Allergy shots can decrease allergic reactions, but have side effects of their own.
Skin Reactions
Local skin reactions commonly occur after allergy shots. Skin may redden, swell around the injection site, become warm to the touch and itch. Localized reactions normally last less than 8 hours, according to the Mayo Clinic, and are usually small, less than 2 centimeters across. A larger reaction area would probably lead your doctor to decrease the dose of your next injection. Treatment includes applying ice to the area and taking antihistamines by mouth. If the area is very swollen and red, topical corticosteroid creams may help decrease inflammation.
Allergic Side Effects
Because the injection being administered contains the same substances a person is allergic to, minor reactions similar to those of an allergic reaction can occur. According to the Mayo Clinic, these include runny nose; sneezing; watery eyes; itching; rash; hives; facial swelling, especially around the eyes and mouth; and difficulty breathing.
Anaphylaxis
Anaphylaxis is a rare allergic reaction to the substance in the injection. The possibility of an anaphylactic reaction is the main reason why most doctors insist that you remain in the office for about 20 minutes after the injection. If anaphylaxis is going to occur after an injection, it will probably occur within 20 minutes. Anaphylaxis causes circulatory collapse, low blood pressure, swelling of the throat and lung collapse, eMedTV explains. Death can occur if treatment isn't promptly administered. Injected adrenaline, also called epinephrine, counteracts the effects of anaphylaxis.
