Hypersensitivity Flashcards
What is hypersensitivity?
How is it provoked?
- an exaggerated, inappropriate adaptive immune response which results in inflammatory reactions and tissue damage
- provoked by many antigens
- will not manifest on 1st contact with antigen, only appears on subsequent contact
How is hypersensitivity classified?
What is each type mediated by?
- 4 types (Type I, II, III, IV)
- in practice these fo not always occur in isolation
- Types I, II, III are antibody mediated
- Type IV - mediated by T cells and macrophages
What is type I hypersenitivity caused by?
List some examples of allergens:
What happens next?
Describe what happens when second encounter with allergen occurs:
Type I (Immediate) Hypersensitivity
- when an individual develops an IgE response to an innocuous environmental antigen
e. g. pollen, house dust mites, animal proteins/hair, milk, nuts, egg, shellfish, latex, some drugs (aspirin, penicillin, LA, sulphonamides)
Next, IgE binds to FcERI on mast cells, as infinity of FcERI receptor is very high for IgE, can remain bound to receptor for around 2 weeks
- second encounter with allergen triggers release of inflammatory mediators –> histamine, prostaglandins, leukotrienes
- produces acute inflammatory reaction within minutes, presenting with symptoms of asthma and rhinitis
What is the treatment for mild Type I hypersensitivity?
- avoidance
- sodium chromoglycate - stabilises mast cells
- anti-histamines
- desensitisation - gradually introduce small amounts of known allergen under very controlled conditions
What can severe Type I reactions result in?
What are the symptoms?
How is this treated?
How is this relevant to dentistry?
Anaphylactic shock, really serious and can be life-threatening
Symptoms:
Wheezing, swelling of face, nausea, dizziness, fainting, low blood pressure
Treatment: adrenaline immediately!
Type I reactions to anaethetics can occur
What is Type II hypersensitivity and when does it occur?
What is the result?
Where does damage occur?
- antibody-dependent cytotoxic hypersensitivity
- occurs in 12-18 hours of exposure to antigen
- occurs when IgG or IgM binds to either a self-antigen or a foreign antigen on cells
- results in phagocytosis, killer cell activity or complement-mediated lysis
Damage is restricted to particular cells/tissues bearing the antigen
Examples of Type II reactions?
Symptoms of Type II reactions?
- blood transfusion reactions where ABO incompatible blood os transferred
- haemolytic disease of the newborn
Symptoms:
fever, hypotension, nausea, vomiting, pain in chest/back
Explain the haemolytic disease of newborn:
- results from incompatibility of Rhesus D blood antigens to RhD-negative mother
- RhD-positive RBC from 1st foetus sensitise mother usually at birth
- if subsequent foetus if RhD-positive, anti-Rh IgG produced by mother crosses placenta and attacks foetus
- RhD-negative mothers are infused with anti-RhD IgG immediately after delivery to eliminate all RhD-positive RBC before immune response occurs, so should she become pregnant with a RhD-positive foetus, she won’t have a problem
When does Type III hypersensitivity occur?
What is this Type mediated by?
What influences rate of clearance?
- occurs within 18-24 hours after exposure to antigen
- Mediated by persistence and deposition of antibody-antigen immune complexes
IC clearance influenced by:
- route of Ag exposure
- dose of Ag
- size of immune complex
- binding of Ag directly to tissues
Once deposited, these IC can cause severe inflammation
When does Type IV hypersensitivity occur?
What are the three types?
What is it mediated by?
Name and describe these phases:
Delayed - occurs in 48-72 hours post Ag exposure
- contact
- tuberculin
- granulomatous: 21-28 days
Mediated by T cells and macrophages
- Sensitisation: Ag encountered by dendritic cell in skin –> migrates to lymph node –> presents Ag to CD4+ T cell
- Elicitation: Ag subsequently encountered –> memory T cells triggered –> inflammation
What can trigger a Type IV hypersensitivity reaction in dentistry?
- amalgam alloy, gold, mercury, resin based materials
What can contact Type IV reactions cause?
Tuberculin?
Granulomatous?
Contact: itchy rash on skin at point of contact with Ag
e.g. nickel, chromate
Tuberculin: skin swelling developing after intradermal exposure
e.g. following injection of tuberculin bacilli
Granulomatous: granuloma formation - aggregation and proliferation of macrophages persisting for weeks
