Allergy (allos = other, different, strange + ergon = activity) is a hypersensitivity disorder of the immune system (Michelle 2010). It is dened as the hypersensitivity to a substance that causes the body to react to any contact with an allergic substance. Allergy is one of the four forms of hypersensitivity and is formally called type I hypersensitivity, which in turn is classied into immediate and late-phase reaction. The immediate hypersensitivity reaction occurs minutes after exposure and includes release of vasoactive amines and lipid mediators, whereas the late-phase reaction occurs 2-4 hours after exposure and includes the release of cytokines. If these reactions are ignored, the entire body gets involved with allergic consequences and then anaphylaxis can take place, which is an acute, systemic reaction that can prove to be fatal. Allergy can be of many types depending on the etiological factor of the outside environment surrounding the human beings (Table 27.1). These factors affect an individual minimally as well as drastically depending on the individual’s genetically triggered immunity level.