Hypersensitivity

Question 1

Type I Hypersensitivity

Answer 1

ALLERGIC TYPE HYPERSENSITIVITY

• Generated in response to a foreign innocuous particle
• Individuals must be sensitised at first before allergy is prompted
• Ag:Ig crosslinking causes immune stimulation
• IgE, bound to the surface of mast cells binds particles stimulating mast cell degranulation and subsequent secretion of histamine. Vasodilation and permeability follow, leading to inflammation.
• Th2 response.
• Example: Asthma

Question 2

Type II Hypersensitivity

Answer 2

ANTIBODY MEDIATED HYPERSENSITIVITY - ANTIBODY DEPENDENT CELLULAR TOXICITY (ADCC)

• IgG, IgM and some IgA
• Antibodies bind to the basement membrane of cells, allowing complement binding (C1q), MAC formation and cell membrane damage follow OR through binding of cytotoxic cells (NK cells)
• Can be directed against foreign particles or in response to self
• Sensitisation
• Example: Rhesus haemolytic disease of the new born

Question 3

Type III Hypersensitivity

Answer 3

ANTIBODY COMPLEX HYPERSENSITIVITY

• Antibody complexes are normally cleared by the liver or spleen (bind to RBCs). However, constant activation can lead to an accumulation of the complexes.
• Ag:Ig complexes are deposited in various tissues types
• The complexes are then able to bind the surface of cells and prompt the complement cascade, cell damage follows.
• Example: Persistent infection, autoimmunity or inhaled antigen

Question 4

Type IV Hypersensitivity

Answer 4

DELAYED TYPE HYPERSENSIVITITY (12h) - TH1

• Th1 stimulation leads to the production of macrophage stimulation cytokines, e.g. IFN-gamma.
• Constant stimulation of macrophages leads to the formation of a giant nucleate cell
• Requires sensitisation to occur
• Example: Contact dermatitis, TB

Question 5

Normal activation of mast cells

Answer 5

Through binding of complement proteins C3a and C5a via CRs

Through binding of epitopes via their sIgE