Allergic reactions (hypersensitivity reactions) are inappropriate responses of the immune system to a normally harmless substance. Usually, allergies make the eyes water and itch, the nose run, the skin itch, rashes develop, and people sneeze.

Symptoms: Sneeze; Itch; RhinorrheaAllergy is also known as a 'hypersensitivity reaction' or a 'hypersensitivity response'. This article uses the terms allergy and hypersensitivity interchangeably. An allergy refers to the clinical syndrome while hypersensitivity is a descriptive term for the immunological process.Hypersensitivity (also called hypersensitivity reaction or intolerance) refers to undesirable reactions produced by the normal immune system, including allergies and autoimmunity. ... Hypersensitivity reactions require a pre-sensitized (immune) state of the host.Allergy and Hypersensitivity. Allergic reactions occur when an individual who has produced IgE antibody in response to an innocuous antigen, or allergen, subsequently encounters the same allergen.Hypersensitivity and inflammatory skin disorders are caused by immune system reactions that involve the skin. ... The immune system reacts to invaders, such as microorganisms, foreign substances, or cancer cells, and triggers inflammation to attack these invaders.