An allergy is an exaggerated immune response or reaction to substances that are generally not harmful.

What are the causes, incidence and risk factors associated with Allergies?
  
What are the symptoms associated with Allergies?
  
What are the treatment options for Allergies?

Causes, incidence, and risk factors

Allergies are relatively common. Both genetics and environmental factors play a role.

Allergy is caused by an oversensitive immune system, which leads to a misdirected immune response. The immune system normally protects the body against harmful substances, such as bacteria and viruses. It reacts to substances (allergens) that are generally harmless and in most people do not cause a problem.

But in a person with allergies, the immune response is oversensitive. When it recognizes an allergen, it releases chemicals, such as histamines. This causes itching, swelling, mucus production, muscle spasms, hives, rashes, and other symptoms, which vary from person to person.

What part of the body is contacted by the allergen plays a role in the symptoms you develop. For example, allergens that are breathed in often cause a stuffy nose, itchy nose and throat, mucus production, cough, or wheezing. A food allergen can cause nausea, vomiting, abdominal pain, cramping, diarrhea, or a severe, life-threatening reaction. Allergies to plants often cause a skin rash. Drug allergies usually involve the whole body and can lead to a variety of symptoms.

Some medical conditions, such as eczema and asthma, are linked to allergies.

Common allergens include pollen, mold, pet dander, and dust. Food and drug allergies are common. Allergic reactions can also be caused by insect bites, jewelry, cosmetics, and other substances.

Some people have allergy-like reactions to hot or cold temperatures, sunlight, or other physical stimuli. Sometimes, friction (rubbing or vigorously stroking the skin) will cause symptoms.

Symptoms

Allergy symptoms vary, but may include:

• Breathing problems
• Burning, tearing, or itchy eyes
• Conjunctivitis (red, swollen eyes)
• Coughing
• Diarrhea
• Headache
• Hives
• Itching of the nose, mouth, throat, skin, or any other area
• Runny nose
• Skin rashes
• Stomach cramps
• Vomiting
• Wheezing

Treatment

The best way to reduce symptoms is to try and avoid what causes your allergies in the first place. This is especially important for food and drug allergies.

The most appropriate medication depends on the type and severity of symptoms. Specific illnesses that are caused by allergies (such as asthma, hay fever, and eczema) may require other treatments. Medications that can be used to treat allergies include:

ANTIHISTAMINES

• Short-acting antihistamines are generally available over-the-counter. They help relieve mild to moderate symptoms but may cause drowsiness. In addition, the short-acting antihistamine diphenhydramine (Benadryl) can affect a child's learning abilities. Loratadine (Claritin) does not tend to cause drowsiness or affect learning in children.
• Longer-acting antihistamines require a prescription. They usually do not interfere with learning. Examples of longer-acting antihistamines include fexofenadine (Allegra) and cetirizine (Zyrtec). Overall, they cause less drowsiness than other medicines, but cetirizine may cause drowsiness in some people.

NASAL SPRAYS

• Nasal corticosteroid sprays are safe and work very well for persons whose symptoms do not go away with antihistamines alone. These prescription medications include fluticasone (Flonase), mometasone (Nasonex), and triamcinolone (Nasacort AQ).
• A newer nasal spray called azelastine (Astelin) is approved for those with seasonal or environmental allergies. It is an antihistamine and not a corticosteroid.
• Cromolyn sodium is a nasal spray (NasalCrom) used to treat hay fever symptoms. (Eye drop versions are available for itchy, bloodshot eyes.)
• Decongestants can help relieve a stuffy nose. However, nasal spray decongestants should not be used for more than several days, because they can cause a "rebound" effect and make the congestion worse. Decongestants in pill form do not cause this problem.

OTHER MEDICINES

• Leukotriene inhibitors are medicines that specifically block the substances that trigger allergies. Montelukast (Singulair) is a prescription medicine approved for those with asthma and indoor and outdoor allergies.
• Allergy shots (immunotherapy) are occasionally recommended if the allergen cannot be avoided and symptoms are hard to control. Allergy shots keep your body from over-reacting to the allergen. Regular injections of the allergen are given, with each dose slightly larger than the previous dose until a maximum dose is reached. They do not work for everybody and require frequent doctor's visits.

Severe allergic reactions (anaphylaxis) require a medicine called epinephrine, which can be life saving when immediately given.