Hypersensitivity reactions

Hypersensitivity reactions excessive undesirable (damaging, discomfort producing and sometimes fatal) reactions produced by the normal immune system. require a pre-sensitized (immune) state of the host. Hypersensitivity reactions can be divided into four types: type I type II type III type IV

Type I Hypersensitivity Reactions It is also known as immediate or anaphylactic hypersensitivity The reaction takes 15-30 minutes from the time of exposure to the antigen. May sometimes be delayed (10-12 hours). The reaction may involve: skin (urticaria and eczema) eyes (conjunctivitis) nasopharynx (allergic rhinitis) bronchopulmonary tissues (asthma) gastrointestinal tract (gastroenteritis) Systemic: Anaphylactic shock from ingested or injected Ags (massive drop in blood pressure. ) Allergens： pollen、dust mite、insects etc mediated by IgE. The primary cellular component is mast cell or basophil. The reaction is amplified and/or modified by other cells such as eosinophils.

Mediators of Immediate Hypersensitivity: It is not clear why some individuals are more prone to type-I hypersensitivity It has been shown that such individuals produce more of TH2 cells that secrete IL-4, IL-5 and IL-13 which in turn favor IgE class switch. IgE has very high affinity for its receptor (Fcε; CD23) on mast cells and basophils. Mediators of Immediate Hypersensitivity: Histamine: Dilates and increases permeability of blood vessels (swelling and redness) increases mucus secretion (runny nose), Causes smooth muscle contraction (e.g. bronchi). Prostaglandins: Contraction of smooth muscle of respiratory system increased mucus secretion. Leukotrienes: Bronchial spasms.

Type II Hypersensitivity Reactions It is also known as cytotoxic hypersensitivity The antigens(allergens) are normally endogenous. Exogenous chemicals (haptens) which can attach to cell membranes can also lead to type II hypersensitivity. Involve activation of complement by IgG or IgM binding to an antigenic cell, ADCC is also involved, through NK cells antigenic cell is then lysed

A. Opsonic phagocytosis Allergen Stimulate Antibody Cell A. Opsonic phagocytosis Combined opsonic activities D. ADCC of NK C. Effect of complement Cell injury ways of type II hypersensitivity

Examples of type II hypersensitivity reaction: 1)Transfusion reaction : hemolysis : mismatch of ABO blood group 2) Hemolytic disease of newborn Mother Rh- : first baby Rh+(Ab), second baby Rh+, fetal RBCs destroyed 3) Autoimmune hemolytic anemia and type II drug reaction 4)Autoimmune thrombocytopenia

Type III Hypersensitivity Reactions Known as immune complex reactions Abs are mostly of the IgG class, although IgM may also be involved. The antigens may be Exogenous: chronic bacterial, viral or parasitic infections Endogenous: non-organ specific autoimmunity: e.g., systemic lupus erythematosus, SLE. Antibody-Antigen immune complexes are deposited in organs, activate complement, and cause inflammatory damage.

immune complex reactions

Common disease of type III hypersensitivity 1. Local immune complex disease Arthus reaction e.g. after vaccination against diphtheria and tetanus 2. Acute systemic immune complex disease   Serum sickness: Anti-serum  Ab+Ag  systemic tissue injury fever, arthritis, skin rash   3. Chronic immune complex disease SLE Rheumatoid arthritis ：RF + IgG  Deposit on synovial membrane

Type IV Hypersensitivity Reactions also known as cell mediated or delayed type hypersensitivity Involve reactions by TD memory cells. First contact sensitizes person. Subsequent contacts elicit a reaction. Inflammation and tissue injury mediated by CD4+Th1 Release cytokines which attract macrophages Cytotoxicity of CD8+CTL

1) Infectious delayed type hypersensitivity Common disease of type IV hypersensitivity 1) Infectious delayed type hypersensitivity Mantoux: OT( Old Tuberculin ) test 2) Contact dermatitis : Paint, drug  red rash, water blister, dermatitis