Saturday, April 18, 2015

Hypersensitivity [ Hypersensitivity Reaction ]

                         Hypersensitivity (also called hypersensitivity reaction or intolerance) is a set of undesirable reactions produced by the normal immune system, including allergies and autoimmunity. These reactions may be damaging, uncomfortable, or occasionally fatal. Hypersensitivity reactions require a pre-sensitized (immune) state of the host.

Types of Hypersensitivity
1. Type 1 - Immediate (or atopic, or anaphylactic)
 Type 1 hypersensitivity reaction is an allergic reaction provoked by re-exposure to a specific type of antigen referred to as an allergen.
 The reaction may be either local or systemic. Symptoms vary from mild irritation to sudden death from anaphylactic shock.
 Treatment usually involves epinephrine, antihistamines and corticosteroids.
 The difference between a normal immune response and a type I hypersensitivity response is that plasma cells secrete IgE. This class of antibodies binds to Fc receptors on the surface of tissue mast cells and blood basophils. Mast cells and basophils coated by IgE are "sensitised".
 Some examples of type 1 hypersensitivity :
■ Allergic asthma
■ Allergic conjunctivitis
■ Allergic rhinitis ("hay fever")
■ Anaphylaxis
■ Atopic dermatitis (eczema)
■ Urticaria (hives)



2. Type 2 - Antibody-Dependent
 In type 2 hypersensitivity reactions, the antibodies produced by the immune response bind to antigens on the patient's own cell surfaces.
 The antigens recognised in this way may either be intrinsic ("self" antigen, innately part of the patient's cells) or extrinsic (absorbed onto the cells during exposure to some foreign antigen, possibly as part of infection with a pathogen).
 IgG and IgM antibodies bind to these antigens to form complexes that activate the classical pathway of complement activation, for eliminating cells presenting foreign antigens (which are usually, but not in this case, pathogens).
 Another form of type 2 hypersensitivity is called Antibody Dependent Cell Mediated Cytotoxicity (ADCC).
 Some examples:
■ Autoimmune hemolytic anemia
■ Goodpasture's syndrome
■ Erythroblastosis Fetalis
■ Immune thrombocytopenia
■ Transfusion reactions
■ Graves' disease
■ Myasthenia gravis
■ Rheumatic fever
■ Hemolytic disease of the newborn

3. Type 3 - Immune Complex
 In type 3 hypersensitivity reactions, insoluble immune complexes (aggregations of antigens and IgG and IgM antibodies) form in the blood and are deposited in various tissues (typically the skin, kidney and joints).
 Some examples:
■ Immune complex glomerulonephritis
■ Rheumatoid arthritis
■ Serum sickness
■ Subacute bacterial endocarditis
■ Symptoms of malaria
■ Systemic lupus erythematosus
■ Arthus reaction

4. Type 4 - Cell-mediated (Delayed-Type Hypersensitivity, DTH)
 Type 4 hypersensitivity reactions are often called delayed type as the reaction takes two to three days to develop. Unlike the other types, it is not antibody mediated but rather is a type of cell-mediated response.
 CD8+ cytotoxic T cells and CD4+ helper T cells recognise antigen in a complex with either type 1 or type 2 major histocompatibility complex.
 The antigen-presenting cells in this case are macrophages which secrete IL-1, which stimulates the proliferation of further CD4+ T cells.
 Some examples:
■ Contact dermatitis (poison ivy rash, for example)
■ Temporal arteritis
■ Symptoms of leprosy
■ Symptoms of tuberculosis
■ Transplant rejection
■ Coeliac disease
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