Immune responses are your body’s natural way of protecting itself. An allergic reaction is defined as hypersensitivity to a particular substance, called allergens. These allergens may cause a hyped reaction via contact with your skin, through inhalation, injection or swallowing. Hence, the symptoms of allergic reactions will typically present themselves in the nose, eyes, skin, and digestive tract. The allergens themselves are not harmful substances. However, in some people, they cause the immune system to overreact. Some of the commonly known allergens are dust, mite, mold, pollen, food, drugs, insect bites, dander, latex and food preservatives.
Types of allergic reactions:
Different types of reactions occur in response to various types of allergens. The symptoms may vary from mild to moderate to severe reaction.
Allergic rhinitis
An allergy causes hay fever or seasonal allergic rhinitis to pollen grains. The symptoms include continuous sneezing, itching, and watery eyes, burning sensation in the throat, running nose and body ache.
Asthma
It is a life-threatening condition characterized by inflammation of airways in the respiratory tract. The inflammation can cause constriction of airways, thus affecting the normal breathing process. The symptoms include difficulty in breathing, tightness in chest, coughing, and wheezing. Asthma is often triggered by hay fever, infections or even heavy exercise.
Anaphylaxis
It is a fatal allergic condition that can be triggered by the reaction to any food, insect bite or medicine. The common symptoms of anaphylaxis are shortness of breath, wheezing, coughing, swelling of tongue and throat, nausea, vomiting, abdominal discomfort, nasal congestion and lightheadedness.
Anaphylactic shock
Anaphylactic shock is defined as a condition in which there is sudden fall in blood pressure. This is due to the excessive enlargement of blood vessels that can cause a dangerous drop in blood pressure. This results in the inadequate supply of blood to various organs in the body. It is a life-threatening condition and requires immediate emergency care.
Treatment:
These days, allergies have become quite common with practically everyone suffering from some form of allergy or the other. 
Mild allergies can be easily managed by avoiding allergens in the first place. Over-the-counter drugs can
help in treating the symptoms. These drugs are known as anti-histamines that reverse the effects of histamine. Some of the other over-the-counter products include nasal balms, decongestants, histamine blockers, saline nasal washes and eye drops.
Sudden allergic reactions can be fatal and require immediate medical attention. Immunotherapy or desensitization is used mainly for severe cases of allergy. It requires a series of injections to desensitize the immune mechanism.