hypersensitivity

Question 1

List and describe the four hypersensitivity types.

Answer 1

I: (Immediate) mediated by IgE
II: (cytolytic or cytotoxic) IgM or IgG binds to Ag on cell/tiss. => complement path. (MAC/opson.) or ADCC
III: (soluble) Ag-Ab complexes deposit in tiss. => complement & inflamm. resp. = infiltration of WBC
IV: (delayed) Th1 => cytokines = accumulation of macrop.. Cytotoxic T cells => local destruction

Question 2

What is Atopy? Name a condition that is atopic

Answer 2

a) IgE-mediated hypersensitivity

b) Atopic dermatitis (aka eczema)

Question 3

What are the steps in an allergic reaction? (3)

Answer 3

1. Sensitisation: IgE produced to allergen (Th2 cytokines => B cell -> plasma => IgE) & binds to receptors on mast cells
2. Activation: re-exposure of Ag => mast cells release granules
3. Effector: inflammatory mediators released from mast cells => complex resp.

Question 4

In what circumstances is IgE production favoured?

Answer 4

• allergens

- parasites

Question 5

Name two mediators in the effector step of an allergic reaction

Answer 5

1. Leukotrienes & prostaglandins > contract broncial & tracheal muscle, vascular permeability & mucus secretion
2. Platelet activating factor (PAF) > plt aggregate => release histamine > vasoconstrict

Question 6

Name two possible clinical consequences of an allergic reaction. (2-3)

Answer 6

• Allergic Rhinitis (hay fever)
• Food Allergies
• Atopic dermatitis (eczema)

Question 7

In a type 2 reaction how is the target cell damaged or destroyed?

Answer 7

• Complement: MAC or opsonisation

- ADCC: NK cells detect Ab bound on self-cells via CD16+ receptor => apoptosis of self-cell

Question 8

How is damaged caused in a type 3 reaction?

Answer 8

activate complement:

• neutrophil infiltrate => release lytic enzymes => tissue damage
• inflammatory resp.: inflamm. cells release cytotoxic mediators

Question 9

Explain how contact hypersensitivity occurs.

Answer 9

1. Contact-sensitising agent (hapten) binds to protein & is taken up by Langerhans cells (DC cells in skin)
2. Sensitisation: Ag processed & presented to Th1 cell in lymph => create memory T cells & migrate twd dermis
3. Elicitation: subsequent exposure to Ag => presented to Th1 cells in dermis => release IFN-gamma
4. keratinocytes => cytokines & chemokines => edema & inflammation

Question 10

Provide an example of the types of hypersensitivity & describe

Answer 10

I: Allergic Rhinitis (hay fever): mast cell mediators => inc. vascular permeability => sneeze & runny nose
II: haemolytic transfusion reaction (HTR) or HDNB. IgM binds to RBC => activate complement = RBC destroyed
III: Arthus rxn: repeated injection of Ag => excess of Ab => insoluble complexes stuck in BV wall
IV: Contact dermatitis: 1º expos.= memory T cell to hapten-carrier complex. 2º expos.= symptoms