Hypersensitivity

Question 1

What is Hypersensitivity and what can result and provoke it?

Answer 1

Hypersensitivity = exaggerated, inappropriate adaptive immune response

Sometimes inflammatory reactions and tissue damage result

Can be provoked by many antigens

Question 2

What is the Classification of hypersensitivity?

Answer 2

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

Question 3

What are the stages of Type I (Immediate) Hypersensitivity?

Answer 3

1. 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

2. IgE binds to FceRI on mast cells
3. Second encounter with allergen triggers release of inflammatory mediators
4. Produces acute inflammatory reaction within minutes with symptoms such as asthma and rhinitis

Question 4

What are the Treatments for mild type I hypersensitivity?

Answer 4

1. Avoidance
2. Sodium chromoglycate - stabilises mast cells, possibly other mechanisms too
3. Anti-histamines
4. Desensitisation

Question 5

What are Severe type I reactions with systemic symptoms are called?

Answer 5

Anaphylactic reactions

Question 6

What are the symptoms and treatment of Anaphylactic reactions?

Answer 6

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

Question 7

What is Type II Hypersensitivity?

Answer 7

Antibody-dependent cytotoxic hypersensitivity

Occurs in 12-18 hours of exposure to antigen

1. Occurs when IgG or IgM binds to either a self-antigen or a foreign antigen on cells
2. Phagocytosis, killer cell activity or complement- mediated lysis

Damage is restricted to particular cells/tissues bearing the antigen

Question 8

What is an example of Type II reactions?

Answer 8

1. Blood transfusion reactions when ABO incompatible blood is transferred
2. 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

Question 9

What is Type III hypersensitivity?

Answer 9

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:

1. Route of Ag exposure eg poor clearance in lung
2. Dose of Ag
3. Size of IC (larger IC fall out of circulation)
4. Binding of Ag directly to tissues

Question 10

What are the 3 types of Type IV hypersensitivity?

Answer 10

Delayed - occurs in 48-72 hours post Ag exposure

Three types:

• Contact 48-72 hours
• Tuberculin 48-72 hours
• Granulomatous 21-28 days

Question 11

What are the 2 phases of Type IV hypersensitivity and what mediates them?

Answer 11

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

Question 12

Answer 12