question: Scientifically, how does a person have an allergy to something, and what occurs in their body during an allergic reaction?
comment: An allergy occurs when a normally harmless substance, called an allergen, is recognized by the body as an invader ,like a bacteria, and the body over reacts with its immune response. The exact response can depend on the allergen and how it enters the body. Hives usually occur from skin contact, runny nose and sneezing are usually breathed in, and anaphylactic shock , a series of reactions, usually results from the allergen entering deep in the body like eating it or injecting it. However, if the allergy is bad enough, something as small as skin contact can be enough. This is a severe simplification and generalization, allergies are a complex aspect of biology we haven't completely figured out. But we have lots of treatments for them and research is improving those treatments.
passages: Common allergens include pollen and certain food. Metals and other substances may also cause problems. Food, insect stings, and medications are common causes of severe reactions. Their development is due to both genetic and environmental factors. The underlying mechanism involves immunoglobulin E antibodies (IgE), part of the body's immune system, binding to an allergen and then to a receptor on mast cells or basophils where it triggers the release of inflammatory chemicals such as histamine. Diagnosis is typically based on a person's medical history. Further testing of the skin or blood may be useful in certain cases. Positive tests, however, may not mean there is a significant allergy to the substance in question.
   An allergic reaction can be caused by any form of direct contact with the allergen—consuming food or drink one is sensitive to (ingestion), breathing in pollen, perfume or pet dander (inhalation), or brushing a body part against an allergy-causing plant (direct contact). Other common causes of serious allergy are wasp, fire ant and bee stings, penicillin, and latex. An extremely serious form of an allergic reaction is called anaphylaxis. One form of treatment is the administration of sterile epinephrine to the person experiencing anaphylaxis, which suppresses the body's overreaction to the allergen, and allows for the patient to be transported to a medical facility.
   If later exposure to the same allergen occurs, the allergen can bind to the IgE molecules held on the surface of the mast cells or basophils. Cross-linking of the IgE and Fc receptors occurs when more than one IgE-receptor complex interacts with the same allergenic molecule, and activates the sensitized cell. Activated mast cells and basophils undergo a process called degranulation, during which they release histamine and other inflammatory chemical mediators (cytokines, interleukins, leukotrienes, and prostaglandins) from their granules into the surrounding tissue causing several systemic effects, such as vasodilation, mucous secretion, nerve stimulation, and smooth-muscle contraction. This results in rhinorrhea, itchiness, dyspnea, and anaphylaxis. Depending on the individual, the allergen, and the mode of introduction, the symptoms can be system-wide (classical anaphylaxis), or localized to particular body systems; asthma is localized to the respiratory system and eczema is localized to the dermis.
   If later exposure to the same allergen occurs, the allergen can bind to the IgE molecules held on the surface of the mast cells or basophils. Cross-linking of the IgE and Fc receptors occurs when more than one IgE-receptor complex interacts with the same allergenic molecule, and activates the sensitized cell. Activated mast cells and basophils undergo a process called degranulation, during which they release histamine and other inflammatory chemical mediators (cytokines, interleukins, leukotrienes, and prostaglandins) from their granules into the surrounding tissue causing several systemic effects, such as vasodilation, mucous secretion, nerve stimulation, and smooth muscle contraction. This results in rhinorrhea, itchiness, dyspnea, and anaphylaxis. Depending on the individual, allergen, and mode of introduction, the symptoms can be system-wide (classical anaphylaxis), or localized to particular body systems; asthma is localized to the respiratory system and eczema is localized to the dermis.
   Section::::Common allergens.
  
  In addition to foreign proteins found in foreign serum (from blood transfusions) and vaccines, common allergens include:
  
  BULLET::::- Animal products
  
  BULLET::::- Fel d 1 (Allergy to cats)
  
  BULLET::::- fur and dander
  
  BULLET::::- cockroach calyx
  
  BULLET::::- wool
  
  BULLET::::- dust mite excretion
  
  BULLET::::- Drugs
  
  BULLET::::- penicillin
  
  BULLET::::- sulfonamides
  
  BULLET::::- salicylates (also found naturally in numerous fruits)
  
  BULLET::::- Foods
  
  BULLET::::- celery and celeriac
  
  BULLET::::- corn or maize
  
  BULLET::::- eggs (typically albumen, the white)
  
  BULLET::::- fruit
  
  BULLET::::- pumpkin, egg-plant
  
  BULLET::::- legumes
  
  BULLET::::- beans
  
  BULLET::::- peas
  
  BULLET::::- peanuts
  
  BULLET::::- soybeans
  
  BULLET::::- milk
  
  BULLET::::- seafood
  
  BULLET::::- sesame
  
  BULLET::::- soy
  
  BULLET::::- tree nuts
  
  BULLET::::- pecans
   Persons with allergy are sensitized to make immune response to several or many proteins contained in one or more of the hundreds of harmless environmental substances, which they take in by inhalation (e.g., house dust mites, pollens, molds, pet animal dander, and other airborne allergens) and ingestion (e.g., wheat (gluten), peanuts, nuts, shellfish, and other food allergens), or through the skin (e.g., bee and fire ant stings, latex gloves). In these individuals, the IgE molecules, both allergen-specific and allergen-nonspecific ones, in their bodies bind to the high affinity IgE receptor (FcεRI) on the surface of mast cells and basophils. Under certain conditions, including but not limited to when the allergenic substances are taken in by a sensitized individual at substantial amounts, the allergenic proteins bind to the allergen-specific IgE bound by FcεRI on the surface of mast cells and basophils and trigger the activation of those inflammatory cells, which release a host of pharmacological mediators, such as histamine, leukotrienes, tryptase, inflammatory cytokines, and others, causing various allergic symptoms/diseases.
   In the early stages of allergy, a type I hypersensitivity reaction against an allergen, encountered for the first time, causes a response in a type of immune cell called a T2 lymphocyte, which belongs to a subset of T cells that produce a cytokine called interleukin-4 (IL-4). These T2 cells interact with other lymphocytes called B cells, whose role is the production of antibodies. Coupled with signals provided by IL-4, this interaction stimulates the B cell to begin production of a large amount of a particular type of antibody known as IgE. Secreted IgE circulates in the blood and binds to an IgE-specific receptor (a kind of Fc receptor called FcεRI) on the surface of other kinds of immune cells called mast cells and basophils, which are both involved in the acute inflammatory response. The IgE-coated cells, at this stage, are sensitized to the allergen.
   In the early stages of allergy, a type I hypersensitivity reaction against an allergen encountered for the first time and presented by a professional antigen-presenting cell causes a response in a type of immune cell called a T2 lymphocyte; a subset of T cells that produce a cytokine called interleukin-4 (IL-4). These T2 cells interact with other lymphocytes called B cells, whose role is production of antibodies. Coupled with signals provided by IL-4, this interaction stimulates the B cell to begin production of a large amount of a particular type of antibody known as IgE. Secreted IgE circulates in the blood and binds to an IgE-specific receptor (a kind of Fc receptor called FcεRI) on the surface of other kinds of immune cells called mast cells and basophils, which are both involved in the acute inflammatory response. The IgE-coated cells, at this stage, are sensitized to the allergen.
   Section::::Pathophysiology.
  
  Section::::Pathophysiology.:Acute response.
   The immune system will produce immunoglobulin E, IgE, antibodies for each allergen. The antibodies will cause cells in the body to produce histamines. These histamines will act on different areas of the body (eyes, throat, nose, gastrointestinal tract, skin or lungs) to produce symptoms of an allergic reaction. 
  
  The allergic response is not limited to a certain amount of exposure. If the body is exposed to the allergen multiple times the immune system will react every time the allergen is present. 
   List of allergens
  
  This is a list of allergies, which includes the allergen, potential reactions, and a brief description of the cause where applicable.
  
  Section::::Allergens.
  
  Section::::Allergens.:Contact.
  
  Many substances can cause an allergic reaction when in contact with the human integumentary system.
  
  Section::::See also.
  
  BULLET::::- Allergic inflammation
  
  BULLET::::- Elimination diet
  
  BULLET::::- Food intolerance
  
  BULLET::::- Oral allergy syndrome
  
  BULLET::::- List of inclusion bodies that aid in diagnosis of cutaneous conditions
  
  BULLET::::- List of cutaneous conditions
  
  BULLET::::- List of genes mutated in cutaneous conditions
  
  BULLET::::- List of target antigens in pemphigus
  
  BULLET::::- List of specialized glands within the human integumentary system
  
  Section::::Further reading.
   When histamine is released from mast cells, it binds to H1 receptors on nerve endings and causes the ocular symptom of itching. Histamine also binds to H1 and H2 receptors of the conjunctival vasculature and causes vasodilatation. Mast cell-derived cytokines such as chemokine interleukin IL-8 are involved in recruitment of neutrophils. TH2 cytokines such as IL-5 recruit eosinophils and IL-4, IL-6, and IL-13, which promote increased sensitivity. Immediate symptoms are due to the molecular cascade. Encountering the allergen a patient is sensitive to leads to increased sensitization of the system and more powerful reactions. Advanced cases can progress to a state of chronic allergic inflammation.
   Aside from these ambient allergens, allergic reactions can result from foods, insect stings, and reactions to medications like aspirin and antibiotics such as penicillin. Symptoms of food allergy include abdominal pain, bloating, vomiting, diarrhea, itchy skin, and swelling of the skin during hives. Food allergies rarely cause respiratory (asthmatic) reactions, or rhinitis. Insect stings, food, antibiotics, and certain medicines may produce a systemic allergic response that is also called anaphylaxis; multiple organ systems can be affected, including the digestive system, the respiratory system, and the circulatory system. Depending on the rate of severity, anaphylaxis can include skin reactions, bronchoconstriction, swelling, low blood pressure, coma, and death. This type of reaction can be triggered suddenly, or the onset can be delayed. The nature of anaphylaxis is such that the reaction can seem to be subsiding, but may recur throughout a period of time.
   Allergic response
  
  An allergic response is a hypersensitive immune reaction to a substance that normally is harmless or would not cause an immune response in everyone. An allergic response may cause harmful symptoms such as itching or inflammation or tissue injury.
  
  Section::::Mechanism.
  
  Allergies are an abnormal immune reaction. The human immune system is designed to protect the body from potential harm and in people who have allergies the immune system will react to allergens (substances that trigger an immune response). 
   Section::::Diagnosis.:Allergy testing.
  
  Allergy testing may help in determining the trigger. Skin allergy testing is available for certain foods and venoms. Blood testing for specific IgE can be useful to confirm milk, egg, peanut, tree nut and fish allergies.
  
  Skin testing is available to confirm penicillin allergies, but is not available for other medications. Non-immune forms of anaphylaxis can only be determined by history or exposure to the allergen in question, and not by skin or blood testing.
  
  Section::::Diagnosis.:Differential diagnosis.
   OAS is a Type 1 or IgE-mediated hypersensitivity, which is sometimes called a "true allergy". The body's immune system produces IgE antibodies against pollen; in OAS, these antibodies also bind to (or cross-react with) other structurally similar proteins found in botanically related plants.
  
  OAS can occur any time of the year but is most prevalent during the pollen season. Individuals with OAS usually develop symptoms within minutes of eating the food.
  
  Section::::Signs and symptoms.
   Based on the symptoms seen on the patient, the answers given in terms of symptom evaluation and a physical exam, doctors can make a diagnosis to identify if the patient has a seasonal allergy. After performing the diagnosis, the doctor is able to tell the main cause of the allergic reaction and recommend the treatment to follow. 2 tests have to be done in order to determine the cause: a blood test and a skin test. Allergists do skin tests in one of two ways: either dropping some purified liquid of the allergen onto the skin and pricking the area with a small needle; or injecting a small amount of allergen under the skin.
   Non-immunologic mechanisms involve substances that directly cause the degranulation of mast cells and basophils. These include agents such as contrast medium, opioids, temperature (hot or cold), and vibration. Sulfites may cause reactions by both immunologic and non-immunologic mechanisms.
  
  Section::::Diagnosis.
  
  Anaphylaxis is diagnosed on the basis of a person's signs and symptoms. When any one of the following three occurs within minutes or hours of exposure to an allergen there is a high likelihood of anaphylaxis:
  
  BULLET::::1. Involvement of the skin or mucosal tissue plus either respiratory difficulty or a low blood pressure causing symptoms
   Hypersensitivities are categorized according to the parts of the immune system that are attacked and the amount of time it takes for the response to occur. The four types of hypersensitivity reaction are: type 1, immediate IgE-mediated; type 2, cytotoxic; type 3, immune complex-mediated; and type 4, delayed cell-mediated. The pathophysiology of allergic responses can be divided into two phases. The first is an acute response that occurs immediately after exposure to an allergen. This phase can either subside or progress into a "late-phase reaction" which can substantially prolong the symptoms of a response, and result in tissue damage.
   Allergic rhinitis is typically triggered by environmental allergens such as pollen, pet hair, dust, or mold. Inherited genetics and environmental exposures contribute to the development of allergies. Growing up on a farm and having multiple siblings decreases the risk. The underlying mechanism involves IgE antibodies attaching to the allergen and causing the release of inflammatory chemicals such as histamine from mast cells. Diagnosis is usually based on a medical history in combination with a skin prick test or blood tests for allergen-specific IgE antibodies. These tests, however, are sometimes falsely positive. The symptoms of allergies resemble those of the common cold; however, they often last for more than two weeks and typically do not include a fever.
   In technical terms, an allergen is an antigen that is capable of stimulating a type-I hypersensitivity reaction in atopic individuals through Immunoglobulin E (IgE) responses. Most humans mount significant Immunoglobulin E responses only as a defense against parasitic infections. However, some individuals may respond to many common environmental antigens. This hereditary predisposition is called atopy. In atopic individuals, non-parasitic antigens stimulate inappropriate IgE production, leading to type I hypersensitivity.
  
  Sensitivities vary widely from one person (or from one animal) to another. A very broad range of substances can be allergens to sensitive individuals.
  
  Section::::Types of allergens.
   BULLET::::- Alder pollen: almonds, apples, celery, cherries, hazel nuts, peaches, pears, parsley, raspberry, strawberry
  
  BULLET::::- Birch pollen: almonds, apples, apricots, avocados, bananas, carrots, celery, cherries, chicory, coriander, fennel, fig, hazel nuts, kiwifruit, nectarines, parsley, parsnips, peaches, pears, peppers, plums, potatoes, prunes, soy, strawberries, wheat; "Potential:" walnuts
  
  BULLET::::- Grass pollen: fig, melons, tomatoes, oranges, celery, peach
  
  BULLET::::- Mugwort pollen : carrots, celery, coriander, fennel, parsley, peppers, sunflower
  
  BULLET::::- Ragweed pollen : banana, cantaloupe, cucumber, green pepper, paprika, sunflower seeds/oil, honeydew, watermelon, zucchini, echinacea, artichoke, dandelions, honey (if bees pollinate with pollen from wild flowers), hibiscus or chamomile tea
   Although allergic contact dermatitis is termed an "allergic" reaction (which usually refers to type I hypersensitivity), its pathophysiology actually involves a reaction that more correctly corresponds to a type IV hypersensitivity reaction. In type IV hypersensitivity, there is activation of certain types of T cells (CD8+) that destroy target cells on contact, as well as activated macrophages that produce hydrolytic enzymes.
  
  Section::::Diagnosis.
   Common foods involved include cow's milk, peanuts, eggs, shellfish, fish, tree nuts, soy, wheat, rice, and fruit. The common allergies vary depending on the country. Risk factors include a family history of allergies, vitamin D deficiency, obesity, and high levels of cleanliness. Allergies occur when immunoglobulin E (IgE), part of the body's immune system, binds to food molecules. A protein in the food is usually the problem. This triggers the release of inflammatory chemicals such as histamine. Diagnosis is usually based on a medical history, elimination diet, skin prick test, blood tests for food-specific IgE antibodies, or oral food challenge.
   BULLET::::- 1965: Production of the first standard series of allergens for epicutaneous testing
  
  BULLET::::- 1966: Introduction of the first commercially available house-dust mite extract by HAL Introduction of the biological standardization for allergen activity determination for each batch
  
  BULLET::::- 1969: Introduction of aluminium hydroxide depot extracts
  
  BULLET::::- 1974: Introduction of HAL ORAL
  
  BULLET::::- 1977: Introduction of product for the diagnosis of type III-allergies; Start operations of HAL Allergy in Germany
  
  BULLET::::- 1980: HALISA - the HAL-ELISA for type III-allergy diagnosis - is introduced
  
  BULLET::::- 1982: HAL allergen extracts are stabilized by epsilon amino caproic acid (EACA)
  