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What is hypersensitivity?
excessive, undesirable (damaging, uncomfortable, and sometimes fatal) reactions produced by the normal immune system
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What does hypersensitivity require?
previous exposure to the offending antigen
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What are exogenous antigens? What are some examples?
- from outside of the body
- examples: flea saliva, dust mites, pollen
- causes allergic dermatitis
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What are endogenous antigens? What are examples?
- "self" antigens that the body does not recognize as "self"
- example: antigens on the patient's own RBCs
- causes autoimmune hemolytic anemia
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How many types of hypersensitivity are there?
4
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Can there be more than one type of hypersensitivity involved in a particular clinical condition?
yes
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What is Type I hypersensitivity called?
immediate or anaphylactic hypersensitivity
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What are type I hypersensitivities mediated by?
IgE and mast cells or basophils
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Describe how type I hypersensitivity works.
- on first contact with specific antigen, IgE is produced in tissues that are exposed to that antigen
- IgE produced on first contact diffuses locally toward mast cells and becomes fixed to the surfaces of the mast cells
- the IgE produced on the first contact is not released into the general circulation
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What happens with type I hypersensitivity on re-exposure to the antigen?
- Ag-Ab complexes form on the surfaces of the mast cells causing them to degranulate
- degranulation of mast cells releases histamine
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What happens when histamine is released?
- causes increased vascular permeability and edema
- causes accumulation of inflammatory cells - mostly eosinophils
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Eosinophilia is common with which type of hypersensitivity?
type I
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What are some examples of type I hypersensitivity?
- hay fever in people
- people allergic to cat dander
- allergic dermatitis
- anaphylactic shock (when there is a massive release of histamine into the general circulation)
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What is term for type II hypersensitivity?
cytotoxic sensitivity
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What is type II hypersensitivity mediated by?
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Are the antigens of type II hypersensitivity endogenous or exogenous?
endogenous
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What does type II hypersensitivity result in?
cell lysis
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What are some examples of type II hypersensitivity?
- hemolytic anemia
- transfusion reaction
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What is another term for type III hypersensitivity?
immune complex hypersensitivity
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What is type III hypersensitivity mediated by?
immune complexes formed by antigens and antibodies (mostly IgG, sometimes IgM)
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Which hypersensitivity can be either generalized or can affect specific organs?
type III hypersensitivity
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What is another term for type IV hypersensitivity?
cell-mediated or delayed type hypersensitivity
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What is type IV hypersensitivity characterized by?
erythema and induration
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What are some examples of type IV hypersensitivity?
- tuberculin reaction
- contact dermatitis - poison ivy
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What are some examples of type III hypersensitivity?
- serum sickness in people given horse serum - old method of passive immunization against tetanus
- systemic lupus erythematosus (SLE) - Ag-Ab complexes deposited in kidneys, joints, skin, etc.
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What does type IV hypersensitivity involve?
- T cells
- monocytes
- macrophages
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During type IV hypersensitivity cells and antigen accumulate at the site to cause _____.
granulomas
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What is a granuloma?
localized nodular inflammation - contains mononuclear inflammatory cells - lymphocytes, macrophages
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