An allergy is defined as an immune response induced by exposure to an allergen. Austrian pediatrician Clemens Pirquet first used the term allergy in 1906. Hypersensitivity reactions can be divided into four groups: type I, type II, type III and type IV, based on the mechanisms involved.
Symptoms of type1 hypersensitivity reactions might vary from mild to severe. Allergen exposure in the beginning might cause mild symptoms, which might get worse when exposed to the allergen. Symptoms of the mild allergic reaction include itching, rashes, hives, nasal congestion, rhinorrhea, sneezing, dyspnea, wheezing and itchy eyes.
Diagnosis of type1 hypersensitivity involves clinical evaluation, evaluating serum IgE levels and complete blood count, skin testing, allergen specific serum IgE testing, ophthalmic testing, nasal and bronchial challenge and provocative testing. Knowing the details of history of the disease is more important than testing procedures. Details about history include duration and frequency of allergic attacks, identifiable triggering factors, relation to settings, responses to attempted treatments and family history of atopic disorders .
Treatment of type1 hypersensitivity reactions include administering antihistamine medications after the allergen is eliminated from the body. Severe hypersensitivity reactions would require a therapy that involves oxygen provision for resolving breathing difficulties. Anaphylactic shock will be managed with intravenous fluids to boost blood pressure. Very severe reactions would require hospitalization. Drugs like mast cell stabilizers, H1 blockers, immunotherapy, and anti-inflammatory corticosteroids and leukotriene inhibitors are also used for treating type1 hypersensitivity reactions [8, 9].
Prognosis of type1 hypersensitivity reactions constitute hives, anaphylaxis, swelling and breathing difficulties that usually improve in few minutes to few hours. Some rashes require many days to heal and a vigorous allergic reaction might require an overnight stay in the hospital. Continued exposure to the allergens might require continued medical therapy. Inhaled or ingested allergens may require a few days to get eliminated from the body .
Allergic or type1 hypersensitivity reactions occur with the interference of IgE. These reactions are caused by opiates, iodinated radiocontrast dye, and vancomycin and appear similar to the urticaria or anaphylaxis. Patients affected by these types of reactions are called Atopic. Atopy is the condition where the patient is genetically predisposed to synthesize IgE antibodies against the allergens.
If allergy is present in a close relative of the patient, then the patient has higher risk of developing allergy. There are no specific causes for allergic hypersensitivity. But, a few reasons are considered as possible causes of this disease. They are pet dander, foods like nuts or shell fish, insect bites, drugs like penicillin or aspirin, some plants and pollen . Relationship between HLA loci and atopy, polymorphism in certain genes that include IgE receptor beta chain, IL-4 receptor alpha chain, CD14, disintegrin are some of the genetic factors that lead to type1 hypersensitivity reactions. Type2 cell-directed immune responses are maintained by the interaction of environmental factors with the genetic factors. The other site specific factors such as adhesion molecules in bronchial epithelium and gastrointestinal tract, and skin, direct the type2 helper cells to the target tissues.
It has been observed that individuals affected by atopic diseases have increased from the year 2000 in the United States. Allergic rhinitis is considered the most widespread affecting 17 to 22 percent of the population. It was estimated that Asthma affected nearly 25 million people in the United States in the year 2010. Predominance of asthma has increased from 7.3 percent in 2001 to 8.4 percent in 2010. Asthma affecting 90 percent of children was found to be due to allergy, while 50 to 70 percent of adults were affected by allergic asthma. According to an epidemiological research program, allergic rhinoconjunctivitis was predominant in children in 56 countries .
Immediate hypersensitivity reactions are driven by IgE. The development of these antibodies is driven by T and B cells. CD4+ T cells are classified into 3 groups called effector T cells, T-regulatory cells and memory T-cells. Effector T-cells are again classified into T-Helper1, T-Helper 2 and T-Helper 17 depending on the cytokines they produced. Cell-mediated immune response is triggered by TH1 cells. TH2 cells synthesize Interleukin-4 and Interleukin-3 that stimulate B-cells producing antigen specific IgE. T-Helper 17 cells produce a set of interleukins to enhance neutrophil inflammation required for immunity in the skin and mucosa, and to produce antimicrobial peptides. The allergen that is ingested is managed by antigen presenting cell, such as B-cell. These cells later get transferred to lymph nodes appearing as naïve TH cells bearing receptors for a particular antigen [4, 5].
People can identify the specific allergens which trigger the inflammatory reaction and learn to avoid them. Types of allergy tests conducted by the specialists include skin testing, blood tests, and checking for the allergic reactions by certain allergens. The identified allergens are then avoided to prevent allergies .
The immune reactions that occur in the human body as protection to the body against diseases can sometimes result in reactions that damage the host itself. These reactions are known as hypersensitivity reactions. Immunopathology is the study of hypersensitivity reactions. There are four mechanisms of hypersensitivity. Type1 is also called immediate hypersensitivity and is IgE-mediated or called common allergy. Type2 reactions are cytotoxic and they are mediated by antibody and cellular mechanisms. Type3 reactions include antigen and antibody complex formations. These complexes attach the red blood cells and help in removing them by phagocytosis, release cytokines, stimulate inflammation in tissue spaces and activate T-cells. Type4 reactions are cell mediated, take more than 12 hours to develop and depend on antigen and T-cell interaction leading to inflammation. Allergy is also a hypersensitivity disorder caused by external antigenic substances. As the types of hypersensitivity vary, the etiology of allergic reactions as example is discussed here .