Allergy is a type of hypersensitivity reaction to a particular substance known as allergen. Hypersensitivity reaction is an unusual response by the immune system inside the body. Person who are more sensitive to a particular substance, have their immune system triggered by a substance that usually does no harm to common people. Substances like pollen, mold, and animal dander are some of the most common allergens known. Allergies are a very common problem, affecting many people.
Generally two risk factors for allergy has been recognized, the host factor and the environmental factors. Hosts include heredity, race, gender, and age, above all heredity had found to be most significant. But the recent studies have shown that the increased number of incidence of allergic disorders that cannot be explained by heredity alone. Second major risk factor is environment, which can be caused due to exposure to various infectious diseases especially during childhood, environmental pollution, dietary changes etc.
Atopic allergy
Allergy is said to be atopic allergy when a person having an inherent tendency to manifest an allergic condition such as asthma, rhinitis, urticaria, and a type of chronic dermatitis (eczematous dermatitis or atopic dermatitis).
Anaphylactic response
Anaphylactic response is a life-threatening response of a sensitized person appears suddenly and severely within few minutes after the exposure of specific allergen and is manifested by difficulty in respiration followed by circulatory collapse. An intense itching sensation (pruritus) and itchy skin eruption (urticaria) with or without subcutaneous oedema are the symptoms seen over the skin of such systemic anaphylactic reactions. Gastrointestinal symptoms include nausea, vomiting, crampy abdominal pain, and diarrhea. This condition needs immediate treatment without which the person’s conditions get worsened.
 
Allergic Reaction
It is still not clearly understood why some people are sensitive to such normally harmless substance, but it is understood that the allergic reaction is an abnormal immune response either accrued or inherent.
Human has a well developed systemic defense mechanism. The main soldiers are immunoglobulin (Ig) known as antibodies (Ab). Ig are of different types and is Y-shaped proteins secreted by a type of white blood cells called plasma cells. These Y-shaped proteins have a capacity to bind the allergen or the foreign bodies entered inside the system and destroy it. The specific antibody associated with allergic reaction is IgE.
Another important member of allergic reaction is Mast Cells and the basophile. These cell carry granules loaded with histamine and heparin. The mast cells are sensitized by a specific IgE- allergen complex when attached on the cell membrane, it releases the stored histamine otherwise called degranulation. Mast cells are located in most of the tissues surrounding blood vessels and nerves, and are mostly found near the areas which are exposed to outside world, such as the skin, mucosa of the respiratory system and internal surfaces of stomach intestine as well as in the mouth, eyes, nose, blood vessels and feet.
Third important member of this reaction is Histamine, an amine produced in by the mast cells or basophils (a type of white blood cells). Histamine is also found in other tissues, like the brain, where it acts as a neurotransmitter and in stomach helps to release digestive enzymes. When a person is exposed to or comes in contact with an allergen, the plasma cells recognise it as foreign meterial and releases the antibody IgE. Then IgE binds the allergen. This bonded IgE- allergen complex further binds the surface of the mast cells to triggers the mast cells to release the histamine stored inside the granules. Certain chemicals such as morphine, Antibiotics hormones, enzymes, pollen extracts, nonpollen extracts, monoclonal antibodies, occupation-related chemical, Hymenoptera venom, polysaccharides, chemotherapy agents, local anesthetics, muscle relaxants, vitamins, diagnostic agents and alkaloids, can sensitise mast cell to release histamine. The allergic reaction is in fact exerted by histamine.
Further allergic reaction exerted by histamine is when it binds to a specific histamine receptors present at a particular site of action. There are 4 types of receptors identified and is named as H1, H2, H3 and H4. When histamine is bonded specifically to H1 receptor causes constriction of respiratory tract, dilation of blood vessel, separation of cells that lines inside the blood vessels and lymphatic vessels which is responsible for hives, and pain and itching due to insect stings, allergic rhinitis, motion sickness, sleep and appetite suppression. H1 receptor is found on smooth muscle, cells that lines inside the blood vessels, and brain tissue.
Common type of Allergic Disorder
Urticaria and Angioedema
Urticaria and Angioedema is a skin manifestations of localized nonpitting oedema and can also occur at mucosal surfaces of the upper respiratory or gastrointestinal tract. Urticaria involves only the outward portion of the skin, presenting as well-circumscribed wheals with abnormal redness of the skin eruption, waxy borders with blanched centers that may combine to become giant wheals. Angioedema is a localized edema occur in the deeper layers of the skin. The condition is said to be acute if recurrence of urticaria and/or angioedema occurs in less than 6 weeks, whereas if attack continues beyond 6 weeks period are said to be chronic.
Allergic Rhinitis/ Hayfever
Allergic rhinitis is characterized by sneezing, Persistent watery mucus discharge from the nose, obstruction of the nasal cavity, itching in nose, throat and eyes, and lacrimation, all these symptoms are seen when a person is exposed to allergen. Hayfever is caused particularly by grass pollen and has similar symptoms of allergic rhinitis.
Contact Dermatitis
Allergic contact dermatitis occur when your skin is exposed to an allergen like by touching clothing, coming in contact with some metal like gold, iron or nickel. Contact dermatitis can cause, burning and itchy rashes, and these may take several days to weeks to cure.
Diagnosis, test and treatment
Diagnosis, test and treatment depends upon type, duration, occurrence, frequency of symptom, occurred either indoor or outdoor, time of worsening of symptoms, how it affects a patients daily activities, health condition etc. Usually skin test and blood test is recommended.
Skin testing is done to identify the substances that cause allergy. Extract of an allergen is applied to the skin by scratching or pricking the skin or sometime inject the allergen under the skin or applied as allergen patch over the skin for a period of time, and wait to notice skin\'s reaction upon it.
Sometimes blood test is done to measure the amount of allergen-specific antibodies especially for Immunoglobulin E (IgE) in the blood. It is done usually with 10 of the most common allergy triggering substance, which includes dust, pet dander, trees, grasses, weeds, and molds. Total white blood cell counts are also done along with eosinophil count.
In conclusion; When the allergen enters into the system, plasma cells releases the antibodies, these antibodies gets attached to the allergens, this allergen- antibody complex further tell mast cells in the body to release certain chemicals called histamine. These chemicals are the one which cause allergy symptoms. Immunoglobulin E (IgE) is an antibody that\'s strongly linked to the body\'s allergy response.