Symptoms of allergies range at a great variety and vary according to the cause of allergy. Many allergens such as dust or pollen are airborne particles. In such cases, symptoms arise in areas in contact with air, such as eyes, nose and lungs. For instance, allergic rhinitis is also known as hay fever and causes irritation of the nose, sneezing, and itching and redness of the eyes. Inhaled allergens can also lead to asthmatic symptoms that are caused by narrowing of the airways like bronchoconstriction and increased production of mucus in the lungs, shortness of breath called dyspnea, coughing and wheezing.

Symptoms of allergy from food include abdominal pain, bloating, vomiting, diarrhea, itchy skin, and swelling of the skin during hives. Food allergies do not usually cause respiratory or asthmatic reactions, or rhinitis. Insect stings, antibiotics, and certain medicines produce a systemic allergic response that is also called anaphylaxis; multiple organ systems can also be affected, including the digestive system, the respiratory system, and the circulatory system.

Depending of the rate of severity of allergy, the symptoms of allergies can lead to coetaneous reactions, bronchoconstriction, edema, hypotension, coma, and even death. This type of reaction can be triggered suddenly, or the onset can be delayed. The severity of this type of allergic response often requires injections of epinephrine, sometimes through a device known as the EpiPen auto-injector. The nature of anaphylaxis is such that the reaction can seem to subside, but may recur throughout a prolonged period of time. Substances that come into contact with the skin, such as latex, are also common causes of allergic reactions, known as contact dermatitis or eczema and are resultant of the symptoms of allergies. Skin allergies frequently cause rashes, or swelling and inflammation within the skin, in what is known as a "wheal and flare" reaction characteristic of hives and angioedema and are also a form of allergy symptoms.

The signs and symptoms of allergy in the form of rhinitis are sneezing, often accompanied by a runny or clogged nose, coughing and postnasal drip, allergic shiners (dark circles under the eyes caused by increased blood flow near the sinuses), itching eyes, nose and throat, the "allergic salute" (in a child, persistent upward rubbing of the nose that causes a crease mark on the nose), watery eyes, conjunctivitis and so on. The signs and symptoms of respiratory allergy are a feeling of tightness in the chest, breathlessness, coughing, difficulty inhaling and exhaling, noisy breathing ("wheezing") and so on. However the symptoms of allergy in case of eczema, contact dermatitis and urticaria include red, itchy skin, inflamed skin and welts (in hives).

If a person experiences more severe symptoms of allergy than what is listed above, they may be in anaphylactic shock. Anaphylactic shock is a medical emergency which is an acute systemic, affecting the whole body because of the allergic reaction. It occurs after exposure to an antigen (allergen) to which a person was previously sensitized. The signs and symptoms of anaphylactic shock include the general symptoms of a common allergic reaction rapidly worsens the situation and the tissues start to swell at the joints. Wheezing, noisy breathing or shortness of breath, rash or redness of the skin, loss of consciousness, anxiety, itching of the skin, headache, nausea and vomiting, abdominal cramps, Diarrhea, low blood pressure, convulsions, hoarseness, itching of the mouth and throat, chest pain and tightness, sweating, cramping of the uterus are some of the symptoms of acute allergy.

Allergens that are more commonly associated with anaphylactic shock are certain insect venoms; drugs such as penicillin; and foods like fish, peanuts, nuts, eggs and seeds. Treatment of anaphylactic shock is first to inject adrenaline (epinephrine) to contract the small blood vessels, raise the blood pressure and expand the airway. This may be followed by vaccination of antihistamines and/or steroids, plus the use of life-support systems.