hypersensitive reactions are excessive immune responses to normally harmless substances that induces inflammation and leads to tissue damage there are four basic types of hypersensitive reactions type 1 reactions are usually known as allergic reactions or atopy and are characterized by an allergen ige antibodies and mast cells when a genetic susceptible person is exposed to a particular antigen now we'll call it the allergen examples could be medicines like penicillins foods like nuts chemicals or pollen throughout different parts of the body some examples could be the gastrointestinal tract the airway or skin ige antibodies will be produced against that allergen for this example we use pollen as the allergen and the airway as the location on a first exposure the allergen is breathed in through the nose or mouth and goes down the airway along the airway we have macrophages or dendritic cells that we call antigen presenting cells these cells will then phagocytose in golf and then the pollen and then present the pollen to t helper cells at local neighboring lymph nodes here the t helper cell selects for a very specific type of b cell that recognizes the pollen this thing goes through clonal expansion into a plasma b cell which produces a lot of ig antibodies against that pollen these antibodies then attach to mast cells which are lined all along the airway on second exposure the pollen comes in comes down the airway and then binds to the antibodies this causes the mast cells to release their granules this process is called degranulation these granules are histamines histamines are then released into the tissue this causes a inflammatory action leading to vasodilation and increased vascular permeability or edema itchiness pain and bronchoconstriction now depending on where this allergy is likely to occur will determine what is the outcome if it's in the upper respiratory tract it will cause hay fever leading to sneezing watery eyes coughing if it's in the gastrointestinal tract it can lead to nausea vomiting diarrhea if it's on the skin can lead to rashes and dermatitis it's in the lower airways it can cause allergy asthma and if it's throughout the body it can cause anaphylaxis