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What is hypersensitivity?
excessive, undesirable (damaging, uncomfortable, and sometimes fatal) reactions produced by the normal immune system
What does hypersensitivity require?
previous exposure to the offending antigen
What are exogenous antigens? What are some examples?
- from outside of the body
- examples: flea saliva, dust mites, pollen
- causes allergic dermatitis
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
How many types of hypersensitivity are there?
Can there be more than one type of hypersensitivity involved in a particular clinical condition?
What is Type I hypersensitivity called?
immediate or anaphylactic hypersensitivity
What are type I hypersensitivities mediated by?
IgE and mast cells or basophils
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
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
What happens when histamine is released?
- causes increased vascular permeability and edema
- causes accumulation of inflammatory cells - mostly eosinophils
Eosinophilia is common with which type of hypersensitivity?
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)
What is term for type II hypersensitivity?
What is type II hypersensitivity mediated by?
Are the antigens of type II hypersensitivity endogenous or exogenous?
What does type II hypersensitivity result in?
What are some examples of type II hypersensitivity?
- hemolytic anemia
- transfusion reaction
What is another term for type III hypersensitivity?
immune complex hypersensitivity
What is type III hypersensitivity mediated by?
immune complexes formed by antigens and antibodies (mostly IgG, sometimes IgM)
Which hypersensitivity can be either generalized or can affect specific organs?
type III hypersensitivity
What is another term for type IV hypersensitivity?
cell-mediated or delayed type hypersensitivity
What is type IV hypersensitivity characterized by?
erythema and induration
What are some examples of type IV hypersensitivity?
- tuberculin reaction
- contact dermatitis - poison ivy
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.
What does type IV hypersensitivity involve?
- T cells
During type IV hypersensitivity cells and antigen accumulate at the site to cause _____.
What is a granuloma?
localized nodular inflammation - contains mononuclear inflammatory cells - lymphocytes, macrophages
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