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Question: Describe the Sequence of Immunological Events that Occur in a Type 1 Hypersensitivity Reaction during both the Primary Exposure to an ntigen (Sensitisation) and for Subsequent Exposures Resulting In Allergic Symptoms. Answer: The immune system of the body is the integral element of normal body functioning whose main function is to protect the body from diseases. However, the protective immune mechanisms might at times lead to detrimental reactions in the body. These reactions are known as hypersensitivity reactions which might be of various forms. Type I hypersensitivity is the allergic reaction that is triggered by reexposure to any antigen that is also to be known as an allergen. Type 1 hypersensitivity reaction involves immunoglobulin E (IgE) mediated release of histamine as well as other mediators from basophils and mast cells. This condition leads to undesirable and excessive reactions that are produced by the normal immune system (Delves et al. 2017). Type 1 hypersensitivity reaction is called as immediate or anaphylactic hypersensitivity. The reaction involves eyes, skin, nasopharynx, gastrointestinal tract, bronchopulmonary tissues. The mechanism of action is marked by the preferential production of IgE which is caused as a response to certain antigens, also referred to as allergens. When an individual gets exposed to such antigens, they preferentially produce an increased amount of TH2 cells. These cells in turn secrete IL4, IL5, and IL 13. This favour IgE class switch in turn. IgE has a high affinity for the receptors on mast cells and basophils. When the same person is exposed to the same allergen, there is a cross-linking of the cell bond IgE, triggering the rapid release of different substances that ar pharmacologically active. Mast cell triggering is mediated by the cross-linking of IgE Fc- receptor. Increased influx of Ca++ precedes degranulation of mast cells that acts as a crucial process. Degranulation is also promoted by ionophores responsible for increasing cytoplasmic Ca++. The agents released mainly include histamine, tryptase, kininogenase, prostaglandins, leukotriene and platelet activation factor (PAF). PAF is responsible for amplifying the reaction leading to platelet aggregation as well as the release of histamine and other agents. Neutrophil chemotactic factors and Eosinophil chemotactic factor of anaphylaxis (ECF-A) attract neutrophils and eosinophils that in turn cause the release of different enzymes with hydrolytic properties. As a result, the cells suffer necrosis. Cyclic nucleotides play a crucial role in modulating the type 1 hypersensitivity reaction. The allergic reaction can be diagnosed with tests that measure total IgE in the body and particular IgE antibodies against the allergens (Abbas, Lichtman and Pillai 2014). Exposure to the different allergens might be ingested, injected, inhaled and coming to direct contact. As a result of the hypersensitivity reaction, there is inflammation at the particular site of since the blood flow to the tissues that are affected is considerably increased. The reactions that the body suffers might be either systemic or local. Symptoms suffered by the person might be mild, such as an irritation, or sudden death as a result of the anaphylactic shock. Anaphylactic shock is often characterised by an unexpected and rapid drop in the blood pressure and difficulties in breathing due to the exposure to the substances. Some common examples of type 1 hypersensitivity reaction include allergic asthma, anaphylaxis, allergic conjunctivitis, eosinophilia and angiooedema. Some frequent antigenic allergens are insect stings, foods, additives, pollens and animal dander (Khalili et al. 2016). References Abbas, A.K., Lichtman, A.H. and Pillai, S., 2014.Basic immunology: functions and disorders of the immune system. Elsevier Health Sciences. Delves, P.J., Martin, S.J., Burton, D.R. and Roitt, I.M., 2017.Essential immunology. John Wiley Sons. Khalili, A., Khayatzadeh, A., Ebrahimi, M., Rafiemanesh, H., Azizi, G. and Movahedi, M., 2016. Evaluation of Tolerance in Patients With Type-1 Hypersensitivity Reaction to Wheat After Oral Immunotherapy.Journal of investigational allergology clinical immunology,26(5), pp.339-340.