Hypersensitivity In general immune system is protective in nature by protecting host body from harmful and infectious foreign microorganisms. But hypersensitivity is an immune disorder where immune system responds more than required and cause harmful effects in host. Hypersensitivity is defined as an exaggeration immune response that results in tissue damage in a sensitized individual on 2nd or subsequent contact with specific antigens. This condition is also called allergy. The first contact with an antigen leads to sensitization the immune system in the host for production of specific allergen produces the manifestation of hypersensitivity. This is known as shocking dose. Basing on the time required for the appearance of the hypersensitivity reactions. They are classified into two types: 1. Immediate hypersensitivity 2. Delayed hypersensitivity Peter Gell and Robert Coombs classified hypersensitivity reactions into 4 types based on the mechanism of pathogenesis: Type 1: IgE mediated hypersensitivity Type 2: Ab dependent cytotoxic hypersensitivity Type 3: Immune complex mediated hypersensitivity Type 4: Cell mediated delayed hypersensitivity The first three types of hypersensitivity reactions are mediated through the production of Antibodies and the fourth type of reaction is mediated through the production of T cells. Type 1: IgE mediated hypersensitivity It occurs in two forms anaphylaxis and atopy. i. Anaphylaxis It is an acute, fatal and systemic form of type I hypersensitivity. The allergens include antibiotics like Penicillin, antitoxin, insect venom etc. On first contact with allergen and lymphocytes are differentiated into plasma cells and produce specific Immunoglobulin IgE also called as ''Reagin antibodies'' which binds to the Fc receptors of mast cells or basophiles and sensitizes these cells that make the individual allergic to the antigen. On the 2nd or subsequent exposure to same allergen the allergen attaches to the surface bound IgE antibodies on the sensitized mast cells or Basophiles and cause cross bridging of adjacent IgE Abs .This leads to the activation and degranulation of mast cells or Basophiles releasing biologically active substances like Histamine, serotonin, Heparin, Leukotrienes, PAF, ECF - A and prostaglandins. These chemical mediators trigger reactions like smooth muscles contraction, vasodilation, increased vascular permeability, respiratory impairment, reduced BP and rapid loss of fluids into tissue spaces. Individual dies within minutes due to asphyxiation, reduced BP, circulatory shock. Diagram illustrating mechanism of Type I Hypersensitivity ii. Atopy It is a chronic, recurrent, non fatal typically localized form of type 1 hypersensitivity. The allergens include inhalants such as pollen grains, fungal spores, animal dander and house dust mites and ingestants like eggs, milk, fruits and fish. Atopy refers to naturally occurring localized hypersensitivity like allergic rhinitis. Atopy is hereditary. The mechanism and effects of atopic allergy is same as anaphylaxis. Hay fever and bronchial asthama are the example for atopy. Hay fever involves upper respiratory tract with clinical symptoms such as congested nasal passage, coughing, sneezing, itch and tearing eyes. Bronchial asthama involves lower respiratory tract. Food allergies are also atopic reactions, with symptoms like hives or urticaria. Type II HS -Antibody dependent cell mediated Cytotoxic HS (ADCC): This type of hypersensitivity is as cytotoxic because it results in the destruction of host cells. During this hypersensitivity IgG or IgM antibodies are directed against cell surface or tissue antigens. They usually stimulate the complement path way and variety of effector cells. The antigen interacts with complement and effector cells through their Fc regions. The good examples of type II hypersensitivity are erythroblastosis foetalis a hemolytic anemia occurs in new born due to Rh incompatibility between Rh negavtive mother and Rh positive foestus. .