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Summary Clinical immunology - hypersensitivity notes
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Introduction, Type I hypersensitivity and Type IV
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HypersensitivityIntroduction
● An ‘allergy’ refers mostly to symptoms whereas hypersensitivity describes the immune response
● Purpose of the immune system is to eradicate pathogens. In doing so, bystander damage of
surrounding tissues can occur from leaked digestive enzymes
● Reactions: skin contact, injection, ingestion, inhalation. Inappropriate response or ‘normal’
response?
● Hypersensitivity is very similar to autoimmunity
● Type I - antibody mediated response - mast cells - immediate - IgE
● Type IV - cell mediated response - delayed hypersensitivity (few days) - lymphocytes/monocytes
- no IgE
Type I hypersensitivity
● Sensitisation - ‘allergen’ processed by APC. Allergen must pass through mucosa initially in order to
be detected
● T cell receptors (TCR) + cytokines → activate B cells
● B cells → plasma cells → differentiation of IgG → IgE
○ IgE binds to most cells and sensitises them
○ Any future exposure to allergen antigen binds to IgE.
● IgE binding → immediate cross linking and opening of ion channels → calcium ion
influx into mast cell → histamine release, leukotriene and prostaglandin
production
○ Causes constriction of airways through smooth muscle contraction
○ This process is called degranulation of the mast cell
○ Examples: hayfever, asthma, eczema, food (nuts/shellfish)
● Hayfever’s symptoms are mainly mucosal and watery eyes with some degree of airway
constriction
○ Allergens: pollen, house dust (DUST MITES)
● Asthma is mainly bronchial airway narrowing. Prevalence mainly in western countries. Developing
in childhood. Allergens: pollen, fur, house dust (DUST MITES), chemicals, weather
○ Extrinsic asthma - atopic - raised IgE conc - allergen induced - family history
○ Intrinsic asthma - idiopathic - serum IgE conc normal but raised eosinophils - no identified
allergen - no family history
○ Treatment through steroids acting as preventers and relievers
○ Nitric oxide is inhaled all the time. Some evidence shows asthmatics have higher conc in
their lungs. Can reducing NO reduce asthma symptoms?
● Eczema is an allergic reaction to S. aureus. Dry, itchy, hot skin → scratching → redness →
cracking, soreness. Atopic eczema most common, runs in families. CLosely linked
to asthma and hayfever.
○ Theory: increased skin protease so skin sheds faster, thus allowing entry of allergens
○ Theory: filaggrin aids skin waterproofing. If mutation in gene then antigens able to
penetrate easier
● Peanut allergy is attributed to the proteins rather than oil. Sensitisation occurs in the mast cells of
throat and gut. Usually begins during childhood. Can be desensitised by gradual exposure over
time.
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