Allergy overview

# Overview
Allergy is a disorder of the immune system that is often called atopy.  Allergic reactions occur to environmental substances known as allergens; these reactions are acquired, predictable and rapid.  Strictly, allergy is one of four forms of hypersensitivity and is called type I (or immediate) hypersensitivity. It is characterized by excessive activation of certain white blood cells called mast cells and basophils by a type of antibody, known as IgE, resulting in an extreme inflammatory response.  Common allergic reactions include eczema, hives, hay fever, asthma, food allergies, and reactions to the venom of stinging insects such as wasps and bees.
Mild allergies like hay fever, are highly prevalent in the human population and cause symptoms such as allergic conjunctivitis, itchiness and runny nose. Similarly, conditions  such as asthma are common, in which allergy plays a major role. In some people, severe allergies to environmental or dietary allergens, or to medication, occur that may result in life-threatening anaphylactic reactions and potentially death.
A variety of tests now exist to diagnose allergic conditions; these include testing the skin for responses to known allergens or analyzing the blood for the presence and levels of allergen-specific IgE. Treatments for allergies include allergen avoidance, use of antihistamines, steroids or other oral medications, immunotherapy to desensitize the response to allergen, and targeted therapy.

# Historical Perspective
Allergy, as a concept, was first defined in the early 1900s. It has later developed into several different disease mechanisms related to disordered activation of immune system. The overall study of allergy, as a concept, drastically evolved in the 1960s with the discovery of immunoglobulin E (IgE) - Kimishige Ishizaka.

# Pathophysiology
The development of allergic response occurs in two phases: acute and late-phase reaction. The body's response varies largely on the type of phase and the advancement of chemical mediation.

# Causes
Allergy causation can categorized as host or environmental factors. Biological factors are strongly familial related. Environmental factors very largely on the type of living environment. Allergies are more common in industrialized countries than in countries that are more traditional or agricultural, and there is a higher rate of allergic disease in urban populations versus rural populations.

# Differentiating Allergy from other Diseases
Allergic response must be differentiated from other similar diseases such as vasomotor rhinitis. Proper diagnosis ensures that the patient receives the most appropriate course of treatment.

# Diagnosis

## History and Symptoms
Allergy has the potential to influence various organ systems differently. Depending of the rate of severity, it can cause cutaneous reactions, bronchoconstriction, edema, hypotension, coma and even death.

# Treatment

## Medical Therapy
Treatment for allergic response is primarily pharmacotherapeutic. Common methodologies include immunotherapy via desensitization or hyposensitization, enzyme potentiated desensitization.

## Secondary Prevention
Once allergies have developed, treating the allergies and carefully avoiding those things that cause reactions can prevent allergies in the future.