Topic 20 - Immune System 1

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These slides are from the previous version of Ask Weber run in 2021

Miscellaneous topics

Wheal and Flare response

  • 'Explain what is meant by the “wheel and flare” response and how it occurs'

What is the Wheal and Flare repsonse?

  • In allergic reactions (a.k.a. Type 1 hypersensitivity reactions), there is a classic appearance comsisting of two components
    1. Wheal: Rapid vascular leakage and mucosal secretions
    2. Flare: Inflammation in surrounding tissue

What happens at a cellular level?

  1. On FIRST exposure
    1. An antigen (e.g. some external substance that induces allergic reactions) enters the skin
    2. T-cells (TH2 and Tfh) respond and stimulate IgE antibody production by B-cells (the cells which produce antibodies)
    3. IgE binds to mast cells (effectively priming mast cells - at this point the IgE isn't in contact with any antigens)
    4. As the mast cells aren't activated, you don't necessarily get an effect on first exposure - this is why sometimes people with peanut allergies do not get a large response on first exposure
  2. on REPEAT exposures
    1. the antigen can bind to the IgE on the mast cell
    2. This activates the mast cell and causes mast cell degranulation (i.e. release of mast cell contents)
    3. Mast cells can vasoactive amines, lipids, cytokines, which in turn can draw in eosinophils, neutrophils, lymphocytes

What causes the wheal and flare?

  • Mast cells release vasoactive amines and proteases when they degranulate
    • histamine is the major amine -> causes small vessel dilation, increases vascular permeability, stimulates smooth muscle contraction
    • protease -> causes damage to local tissue
    • Arachidonic acid -> metabolises into prostaglandins -> causes vascular dilation
      • also metabolises into leukotrienes -> stimulates smooth muscle contraction
    • cytokines
  • The overall effect is that:
    1. Blood vessels dilate
      • this increases blood flow to the area, resulting heat and redness (i.e. rubor, calor)
    2. Blood vessels become leaky
      • this results in swelling in the area as fluid enters interstitial tissue (i.e. tumor)
    3. stimulation of pain receptors in area
  • Wheal is the area of concentrated local oedema that occurs, flare is the spreading out as nearby vessels also vasodilate (histamines, etc. in the tissue can result in local capillary networks to become leaky, etc.)

Tissue-resident cells

  • 'Identify major tissue-resident cells and where they are found and their function in the immune system'