IMMUNOLOGY

Detailed explanation-1: -Hypersensitivity reactions are exaggerated or inappropriate immunologic responses occurring in response to an antigen or allergen. Type I, II and III hypersensitivity reactions are known as immediate hypersensitivity reactions because they occur within 24 hours of exposure to the antigen or allergen.

Detailed explanation-2: -An allergic reaction occurs when the immune system overreacts to exposure to antigens in the environment.

Detailed explanation-3: -There are four types of hypersensitivity reactions: type I is immunoglobulin E, or IgE mediated; type II is mediated by antibodies that activate cellular cytotoxicity, type III is mediated by immune-complexes, and type IV is a delayed T cell mediated hypersensitivity reaction.

Detailed explanation-4: -Antigens that can induce an allergic response are called allergens, and they are often derived from non-infectious or non-microbial sources.

Detailed explanation-5: -Hypersensitivity reactions are an overreaction of the immune system to an antigen which would not normally trigger an immune response.