Hypersensitivity Flashcards
What is Hypersensitivity and what can result and provoke it?
Hypersensitivity = exaggerated, inappropriate adaptive immune response
Sometimes inflammatory reactions and tissue damage result
Can be provoked by many antigens
What is the Classification of hypersensitivity?
Classified into four types (Type I, II, III and IV)
In practice these do not always occur in isolation
- Types I, II, III - Antibody mediated
- Type IV - mediated by T cells and macrophages
What are the stages of Type I (Immediate) Hypersensitivity?
- IgE response to innocuous environmental antigens
- pollen
- house-dust mites
- animal proteins/hair
- certain foods – milk, nuts, egg, shellfish
- latex – gloves, dressings (rare)
- some drugs – aspirin, penicillin, sulphonamides, local anaesthetics
- IgE binds to FceRI on mast cells
- Second encounter with allergen triggers release of inflammatory mediators
- Produces acute inflammatory reaction within minutes with symptoms such as asthma and rhinitis
What are the Treatments for mild type I hypersensitivity?
- Avoidance
- Sodium chromoglycate - stabilises mast cells, possibly other mechanisms too
- Anti-histamines
- Desensitisation
What are Severe type I reactions with systemic symptoms are called?
Anaphylactic reactions
What are the symptoms and treatment of Anaphylactic reactions?
Symptoms:
- wheeze, swelling of face
- and laryngopharynx,
- nausea, dizziness, fainting,
- low blood pressure
Treatment:
Must treat with ADRENALINE immediately
Dentists
- type I reactions to anaesthetics can occur
- Injection can result in generalised anaphylaxis
- Must ALWAYS have adrenaline near
- Type I reactions to latex also possible
What is Type II Hypersensitivity?
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
- Phagocytosis, killer cell activity or complement- mediated lysis
Damage is restricted to particular cells/tissues bearing the antigen
What is an example of Type II reactions?
- Blood transfusion reactions when ABO incompatible blood is transferred
- Haemolytic disease of the 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 is RhD-positive, anti-Rh IgG produced by mother crosses the placenta and attacks foetus
- RhD-negative mothers infused with anti-RhD IgG immediately after delivery - eliminates all RhD-positive RBC before immune response occurs
What is Type III hypersensitivity?
Occurs within 18-24 hours
Example: Extrinsic allergic alveolitis. Eg pigeon fancier’s lung, farmer’s lung
Mediated by persistence and deposition of antibody-antigen immune complexes
IC clearance influenced by:
- Route of Ag exposure eg poor clearance in lung
- Dose of Ag
- Size of IC (larger IC fall out of circulation)
- Binding of Ag directly to tissues
What are the 3 types of Type IV hypersensitivity?
Delayed - occurs in 48-72 hours post Ag exposure
Three types:
- Contact 48-72 hours
- Tuberculin 48-72 hours
- Granulomatous 21-28 days
What are the 2 phases of Type IV hypersensitivity and what mediates them?
Mediated by T cells and macrophages
Two phases -
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 are triggered
- Inflammation
