An allergic reaction is essentially an overreaction of the body's immune system to foreign substances called allergens. For people who suffer from allergies, the immune response to certain allergens can be very uncomfortable, or even life threatening. Difficulty swallowing can be a symptom or result of allergic reactions, as well as a side effect of allergy medications.
Anaphylaxis is an extreme allergic reaction that occurs when antibodies release very large quantities of histamine in response to an allergen. A person suffering from anaphylaxis often experiences some degree of swelling in the throat, making it difficult both to swallow and to breathe.
Other typical symptoms of anaphylaxis include cramping, abdominal pain, dizziness, confusion, diarrhoea, vomiting, hives, dilated blood vessels, lowered blood pressure and, in worst-case scenarios, shock. A person who lapses into anaphylactic shock may quickly become unconscious, and will die if untreated. Epinephrine (adrenalin) injection is the only quick, effective treatment for anaphylaxis.
People who have allergies to certain foods are most likely to experience difficulty swallowing and other anaphylactic symptoms when they come in contact with these allergens. Common food allergens include nuts, dairy products, shellfish, sesame seeds, egg whites, and legumes (especially peanuts).
Although pollen, dust and other inhaled allergens rarely cause reactions beyond itchy eyes and a runny nose, some people with hay fever may experience symptoms of anaphylaxis---most commonly swelling of the throat and difficulty swallowing---when they eat certain fruits and vegetables. The body mistakes proteins in the fruit or vegetable for similar proteins in pollen, and this triggers an allergic reaction known as pollen-food allergy syndrome.
Other allergens that have been known to cause anaphylactic symptoms include latex, bee stings and penicillin.
Although non-anaphylactic allergic reactions do not usually cause swallowing difficulties, some of the medications used to treat such reactions may bring about these symptoms. Antihistamines, which are the active ingredient in most allergy medicines, cause dry mouth, or xerostomia, in many patients. This dryness may cause swallowing difficulties.
Other Indirect Causes
A number of allergic reactions---particularly to hay fever, dust and other inhaled allergens---can cause sinus problems, itchy or sore throat, and persistent coughing. These conditions may lead to difficulty or discomfort when swallowing, although the allergic reaction itself does not directly cause swallowing problems.