Allergy and Hypersensitivity Flashcards
4 types of hypersensitivity reaction
Type I:
- Classical allergy,
- Mediated by the inappropriate production of specific IgE antibodies to harmless antigens
Type II:
- Caused by IgG and IgM antibodies – that bind to antigens/cells/tissues and cause cell/tissue damage
Type III:
- Caused by antibody-antigen complexes being deposited in tissues
- Where they actiate the complement system and cause inflammation
Type IV:
- A delayed type hypersensitivity reaction caused by Th cells traveling to the site of the antigens
- Recruiting macrophages and causing inflammation
Allergen definition
Antigens that produce allergic reactions (e.g. peanuts, pollen or penicillin)
Sensitization (type I)
Term to describe the initial event that lead to the specific IgE being developed for that allergen
1) CD4 cells recognise allergen
2) they proliferate and differentiate into T Helper 2 cells
3) These Th3 cells release IL-4, that stimulates the production of IgE by B cells specific to that allergen
4) The IgE then circulates the blood and binds to mast cells
Type 1 hypersensitivity
IgE Mediated - traditional allergy
Examples:
- Food or drug allergy
- Asthma
- Allergic rhinitis
- Hayfever
- Eczema
Involves Sensitisation before allergic response
Allergic response involves activation of mast cells (causing degranulation)
Allergic response in Type I Hypersensitivity
Involves activation of mast cells (after sensitisation)
1) On On re-exposure to the allergen, it binds to the IgE and causes mast cell degranulation, releasing cytokines including histamine and TNF-α.
2) Histamine causes vasodilation, increased vascular permeability and broncho-constriction, causing symptoms of allergy (itch, flushing, rash, angioedema and wheeze). This happens within minutes of exposure to the allergen.
3) TNF-α causes a localised inflammatory process at the site of exposure. This takes a few hours and is called the late phase reaction.
4) The allergic response can vary from mild reactions involving itch, mild swelling and hives to severe reactions called anaphylaxis that can lead to systemic shock (from severe vasodilation) and complete airway closure from broncho-constriction and oedema.
5) Allergic responses to allergens tend to get worse on repeat exposures due to increased sensitisation.
What is responsible for the late-phase reaction in T1 hypersensitivity?
TNF-α
How can the diagnosis of anaphylaxis be diagnosed?
Mast cell tryptase measurements after an anaphylactic reaction
Type 2 hypersensitivity reaction (with 3 examples)
Involves IgM and IgG antibodies
Binds to antigens on cells or tissues that result in a reaction that is damaging to the person
Examples are:
- Goodpastures
- Haemolytic Disease of the Newborn
- Blood transfusion reactions
Goodpastures syndrome
Type II Hypersensitivity
Antibodies specific to a type of collagen in the glomerular basement membrane in the kidneys and the lungs –>
leads to inflammation and destruction of the basement membrane leading to –>
Pulmonary haemorrhage and kidney failure
Haemolytic Disease of the Newborn
Type II Hypersensitivity
When a rhesus negative mother has a rhesus positive baby, exposure to the babies blood during birth will cause the mother to produce IgG to rhesus.
If she has another rhesus positive baby, that IgG will cross the placenta into the babies bloodstream and cause haemolysis of the babies red blood cells.
Blood transfusion reactions
When blood is transfused and the ABO group of the donor does not match the recipient, the antibodies in the recipients blood attack the donors blood causing haemolysis of the donor red blood cells, rapidly releasing the contents of those cells and causing a toxic reaction.
Type II Hypersensitivity
Type III Hypersensitivity:
Antibody-antigen immune complexes
Mainly IgG and IgM mediated
Complexes become deposited in tissues
– Activate complement system and cause inflammation
2 Examples:
- Rheumatoid arthritis
- Farmers lung
Main difference between TII/III hypersensitivity reactions:
Type II it is the antibodies binding to the target that causes inflammation and damage of the target
Whereas in type III, the antibodies bind to antigens, and it is the antibody-antigen complexes that travel to their target organs where they cause inflammation and damage.
Rheumatoid arthritis
Type III Hypersensitivity
Rheumatoid factor is IgM antibody that recognises IgG antibodies as an antigen, specifically the Fc portion
IgM against IgG
This leads to formation of antibody-antigen complexes in the blood. These become deposited in joints, skin, lungs and other organs where they activate the complement system and lead to CHRONIC INFLAMMATION.
Farmers lung:
Type III Hypersensitivity
Mould and hay spores are breathed into the lungs.
Antibodies against the mould or hay antigens form antibody-antigen complexes.
These are deposited in the lung tissues and alveoli where they activate the complement system and lead to inflammation of the lung tissue.
