Hypersensitivity Flashcards
What is an antigen
Any molecule or molecular structure that can be recognised by an antibody or the adaptive immune system
What is type I hypersensitivity?
Also called immediate or anaphylactic hypersensitivity
An allergic reaction provoked by re-exposure to a specific type of antigen referred to as an allergen
Includes asthma, allergic rhinitis, atopic dermatitis
What are type I responses mediated by?
Antigen specific IgE antibodies
Non allergic people tend to only make these in response to parasitic infections or potent venoms
However allergic Individuals produce antibodies against common multivalet environmental antigens (allergens)
Eg. Nuts, birch trees, dogs, penicillin and dust mites
How are allergies diagnosed?
Skin prick tests - exposing the skin to a small amount of allergen
How does the respond In allergic people to allergens work?
The generation of type 2 helper CD4 T cells, and B cell helper follicular CD4 T cells which produce the type 2 cytokines IL-4 and IL-13
These cause B cells to switch on and produce antigen specific IgE
Unlike other antibodies IgE is very rarely found in circulation, once produced it is rapidly bound to the surface of innate immune cells, especially mast cells and basophils. Once bound these are degranulated very quickly
This occurs much faster than in normal inflammation
What is the result of type I hypersensitivity reactions?
Release of histamine, a host of cytokines that recruit other cells and promote further Th2 differentiation, and highly active smooth muscle contracting molecules such as leukotrienes and prostaglandins
What is type II hypersensitivity?
Aka antibody mediated cytotoxic hypersensitivity
Involves the destruction of cells by IgG or IgM antibodies bound to antigens on the surface of cells
Eg. Wrong blood group blood transfusions, haemolytic disease of newborns, Graves’ disease
How can the antibodies involved in type Ii hypersensitivity reactions cause disease?
- Anti-receptor activity - blocking or activating its function
- Antibody dependant cell mediated cytotoxicity
- classical activation of the complement cascade
What is the complement cascade?
A complex process by which antibody on the surface of cells is recognised by the complement components, Ultimately leading to the formation of the membrane attack complex (MAC) in the surface of the cell, and cell death due to the loss of osmotic integrity.
Activation of the complement pathway also leads to inflammation, opsonisation and recruitment and activation of immune cells
What is type III hypersensitivity?
Aka immune complex driven disease
Immune complexes - non cell bound antigen antibody complexes that are normally cleared Through the immune system. However if these compexws can’t be cleared (eg if they are the result of antibodies reacting agains self antigens such as DNA) the immune complexes are deposited in the blood vessel walls and tissues. This leads to inflammation and tissue damage
What is type IV hypersensitivity?
Aka delayed type or T cell mediated hypersensitivity
Primarily initiated by T cellsas with the other ones there needs to be a sensitivity phase where the antigen is presented to naive T cells by dendritic cells generating T memory cells.
Because the cell mediaee tree d response is slightly slower than that if antibodies, type IV hypersensitivity reactions peak around 2-3 days after inflammation
What are some examples of type IV hypersensitivity reactions?
Contact dermatitis caused by poison ivy - urushiol acts as a hapten (binds to proteins on the skin), drives a T helper 1 reaponse (but due to small nature doesn’t generate an antibody reaponse)
Please look at the note bank for this one!!!
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