An allergy is a hypersensitivity disorder of the immune system.  Allergic reactions occur when a person’s immune system reacts to normally harmless substances in the environment. A substance that causes a reaction is called an allergen. These reactions are acquired, predictable, and rapid. Allergy is one of four forms of hypersensitivity and is formally called type I (orimmediate) hypersensitivity. Allergic reactions are distinctive because of excessive activation of certain white blood cells called mast cells and basophils by a type of antibody called Immunoglobulin E (IgE). This reaction results in an inflammatory response which can range from uncomfortable to dangerous.allergies

Mild allergies like hay fever are very common in the human population and cause symptoms such as red eyes, itchiness, and runny nose, eczema, hives, or an asthma attack. Allergies can play a major role in conditions such as asthma. In some people, severe allergies to environmental or dietary allergens or to medication may result in life-threatening reactions called anaphylaxis. Food allergies, and reactions to the venom of stinging insects such as wasps and bees are often associated with these severe reactions.

[2]A variety of tests exist to diagnose allergic conditions. These include placing possible allergens on the skin and looking for a reaction such as swelling.  Immunotherapy uses injected allergens to desensitize the body’s response.

Signs and Symptoms

  • Nose: Swelling of the nasal mucosa (allergic rhinitus)
  • Sinuses: allergic sinusitis
  • Eyes: redness and itching of the conjunctiva (allergic conjunctivitis)
  • Airway: Sneezing, coughing, broncho-constriction, wheezing and dyspnea, sometimes outright attacks of asthma, in severe cases the airway constricts due to swelling known as laryngeal edema
  • Ears: feeling of fullness, possibly pain, and impaired hearing due to the lack of eustachian tube drainage.
  • Skin: rashes, such as eczema and hives (urticaria)
  • Gastrointestinal Tract:  abdominal pain, bloating, vomiting, diarrhea

Many allergens such as dust or pollen are airborne particles. In these cases, symptoms arise in areas in contact with air, such as eyes, nose, and lungs. For instance, allergic rhinitis, also known as hay fever, causes irritation of the nose, sneezing, itching, and redness of the eyes. Inhaled allergens can also lead to asthmatic symptoms, caused by narrowing of the airways (broncho-constriction) and increased production of mucus in the lungs, shortness of breath (dyspnea), coughing and wheezing. 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, antibiotics, and certain medicines 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.


One of the most common food allergies is a sensitivity to peanuts. Peanut allergies may be extremely severe, but can sometimes be outgrown by children school-age.[13] Tree nuts, including pecans, pistachios, pine nuts, and walnuts, are another common allergen. Sufferers may be sensitive to one, or many, tree nuts.[14] Also seeds, includingsesame seeds and poppy seeds, contain oils where protein is present, which may elicit an allergic reaction.[14]Egg allergies affect one to two percent of children but are outgrown by about two-thirds of children by the age of 5.[15] The sensitivity is usually to proteins in the white rather than the yolk.[14]Milk, from cows, goats, or sheep, is another common allergy-causing food, and many sufferers are also unable to tolerate dairy products such as cheese. Lactose intolerance, a common reaction to milk, is not in fact a form of allergy. A small portion of children with a milk allergy, roughly ten percent, will have a reaction to beef. Beef contains a small amount of protein that is present in cow’s milk.[16]Other foods containing allergenic proteins include soy, wheat, maize, fish, shellfish, fruits, vegetables, spices, synthetic and natural colors, chicken, and chemical additives.

Non-Food Proteins

Latex can trigger an IgE-mediated cutaneous, respiratory, and systemic reaction. Latex and banana sensitivity may cross-react; furthermore, patients with latex allergy may also have sensitivities to avocado, kiwifruit, and chestnut. These patients often have perioral itching and local urticaria. Only occasionally have these food-induced allergies induced systemic responses. Researchers suspect that the cross-reactivity of latex with banana, avocado, kiwifruit, and chestnut occurs because latex proteins are structurally homologous with some plant proteins.

Toxins Interacting With Proteins

Another non-food protein reaction, urushiol-induced contact dermatitis, originates after contact with poison ivy, eastern poison oak, western poison oak, or poison sumac.Urushiol, which is not itself a protein, acts as a hapten and chemically reacts with, binds to, and changes the shape of integral membrane proteins on exposed skin cells.  The resulting dermatological response to the reaction between urushiol and membrane proteins includes redness, swelling, papules, vesicles, blisters, and streaking

Genetic Basis

Allergic diseases are strongly familial: identical twins are likely to have the same allergic diseases about 70% of the time; the same allergy occurs about 40% of the time innon-identical twins. Allergic parents are more likely to have allergic children, and their allergies are likely to be more severe than those from non-allergic parents.

Hygiene Hypothesis

Allergic diseases are caused by inappropriate immunological responses to harmless antigens driven by a TH2-mediated immune response.  In other words, individuals living in too sterile an environment are not exposed to enough pathogens to keep the immune system busy. Since our bodies evolved to deal with a certain level of such pathogens, when they are not exposed to this level, the immune system will attack harmless antigens and thus normally benign microbial objects — like pollen — will trigger an immune response.