L18 Atopy, allergy and dth 2 Flashcards
Detection of allergen-specific IgE in vivo: skin testing
Skin prick testing
- Allergen extract applied as drops
- Top layers of epidermis punctured with lancet
- A wheal with flare response after 15 mins is positive
- Result needs interpretation in clinical context
Detection of allergen-specific IgE in vitro
- Performed by radioallergosorbant (RAST) assay a very long time ago
- Now usually by ELISA, but term ‘RAST’ still widely used clinically
Process of allergen-specific IgE in vitro
- Plastics coated with purified allergen of interest
- Incubate with patient serum
- IgE antibodies in sera of sensitised patient bind to allergens
- Immobilised IgE antibodies detected with polyclonal anti-IgE detection antibody
Treatment of allergy: pure symptom relievers
- Nasal decongestants
- B2 agonists
- Epinephrine
Nasal decongestants
eg oxymetazoline
- Act on alpha1 adrenoreceptors to cause vasoconstriction
- Only for short-term use topical and systemic
B2 agonists
eg salbutamol
- Act on lung B2 adrenoreceptors, cause smooth muscle relaxation
Epinephrine
- Systemic adrenergic effects oppose vasodilatation and bronchoconstriction
Treatment of allergy: drugs acting on early-phase mediators
- Mast cell stabilisers
- H1 antihistamines
- Leukotriene receptor antagonists
Mast cell stabilisers
eg sodium cromoglycate
- Reduce mast cell degranulation by unknown mechanism
- Not orally absorbed - topical use only
- Short half-life requires frequent dosing
- Main benefit is steroid-free, but efficacy very poor
H1 antihistamines
- Inverse agonists at H1 histamine receptor
- Best used before exposure to allergen
1st gen H1 antihistamines
Chlorpheniramine
- considerable sedation, drug interactions
2nd gen H1 antihistamines
Cetirizine, loratidine, desloratidine, fexofenadine
- No/minimal sedation, once-daily
Leuktriene receptor antagonists
- Only UK drug is montelukast
- Effective in reducing early allergic responses, but inferior to H1 antihistamines
- Unlike anti-histamines, beneficial in chronic asthma, which is the main indication for their use
Treatment of allergic disease: corticosteroids
- Steroid receptors are found in the cytoplasm complexed with a heat-shock protein Hsp90
- Steroids cross the cell membrane and bind to the steroid receptor complex, releasing Hsp90
- The steroid:receptor complex can now cross the nuclear membrane
- In the nucleus, the steroid receptor binds to specific gene regulatory sequences and activates transcription
How do steroids reduce immune activation
- Steroids reduce immune activation by altering gene expression in numerous cell types, including T cells, B cells and cells of the innate immune system
- Their onset of action is delayed and they must be taken regularly
Examples of corticosteroids
Inhaled eg beclometasone, fluticasone
Nasal eg beclometasone, mometasone, fluticasone
Also for skin(eg hydrocortisone) and ophthalmic drops. Topical preparations may cause local and even systemic side effects
Oral, intravenous and depot preparations available
Treatment of allergic disease: omalizumab
- Omalizumab is a monoclonal antibody directed against IgE, used for atopic asthma (amongst other things)
Treatment of allergic disease; allergen-specific immunotherapy
- Allergen doses administered by subcutaneous injection or sublingually
- Provide long-term protection
- Mainly venom allergy and rhinitis
Allergen-specific immunotherapy - multiple immunological effects
- Induce regulatory T cell responses to allergens
- Reduce Th2 responses
- Induce allergen-specific IgG antibodies
- Reduction in mast cell responsiveness
- Reduce allergen-specific IgE levels
Early effect of allergen-specific immunotherapy
Desensitization of mediator release from mast cells and basophils to allergen exposure is the first change noted with the initiation of allergen immunotherapy
Intermediate effect of allergen-specific immunotherapy
As allergen dosing is increased in the course of immunotherapy, the next most notable response is a change in T cell subset distribution with the generation of allergen specific T regulatory (T reg) cells and a decrease in Th2 cells
Repeated allergen exposure stimulates IL-10 and TGF-β expression by allergen-specific, inducible, type 1, peripheral T regulatory (Tr1) cells, which act in an autocrine fashion to further activate these Tr1 cells and initiate peripheral tolerance
Late effect of allergen-specific immunotherapy
The late changes include decrease in IgE production by B cells and an increase in IgG4 and IgA serum levels. These changes are responsible for the clinical effect of ameliorating allergy symptoms.
Key overall change as a result of allergen immunotherapy
One of the key changes resulting from allergen immunotherapy is the generation of T-reg cells resulting in peripheral tolerance
The newly generated T reg cells from immunotherapy have several suppressive mechanisms including the expression of two cytokines, IL-10 and TGF-β.
Type IV, delayed-type hypersensitivity
Mediated by antigen-specific effector T cells
- Antigen-specific - implies that a specific antigen stimulus is required, which is then processed and presented to relevant T cells which are responsible for reaction
- Effector T cell - T cells that have previously met antigen and are ‘primed’ to produce a rapid, robust response
- Because it takes time to process and present antigen, these reactions do not develop for at least 24 hrs following exposure
Contact dermatitis: the prototype type IV hypersensitivity
- Sesitising agents are typically highly reactive small molecules which can penetrate skin
- These react with self proteins to create protein-hapten complexes that are picked up by langerhans cells, which migrate to regional lymph nodes
What are hapten molecules
Hapten = small molecule which cannot produce an immune response by itself, but can bind to protein to alter its immunogenicity
Contact dermatitis - sensitisation
- The langerhans cells process and present the antigen together with MHCII
- In some susceptible individuals, the complexes are recognised as foreign
- The activated T cells then migrate to the dermis
Contact dermatitis - elicitation
Antigen is processed by tissue macrophages and stimulates Th1 cells
- Chemokines recruit macrophages
- Th1 cells secrete IFN gamma: increases expression of vascular adhesion molecules, activates macrophages
- TNF alpha/beta: local inflammation
Poison ivy
- Pentadecacatechol is a poison ivy lipid that may cross the skin and modify intracellular proteins
- These proteins are processed and presented with MHC1 to CD8 T cells which then cause contact dermatitis
Patch testing for contact dermatitis
- Antigen-impregnated patch placed on back
- Nickel, chrome, cobalt, epoxy resin, lanolin etc
- Results read after 2 days
Contact dermatitis vs type 1 allergy - clinical features
Type 1 - various features consistent with mast cell degranulation
Type IV - eczematous skin reaction
Contact dermatitis vs type 1 allergy - temporal aspects
Type 1 - closely follows exposure then improves fairly rapidly
Type IV - delay between exposure and symptoms
Contact dermatitis vs type 1 allergy - causative agent
Type 1 - almost always naturally-occurring protein or closely related to one
Type IV - various, often synthetic molecules
Contact dermatitis vs type 1 allergy - effector mechanism
Type 1 - allergen-specific IgE, mast cell degranulation
Type IV - antigen-specific effector Th1 cells
Contact dermatitis vs type 1 allergy - assessment
Type 1 - allergy clinic - history, skin prick testing, serology for allergen-specific IgE
Type IV - dermatology clinic (UK) or allergy clinic (europe), history and patch testing
Contact dermatitis vs ty[e 1 allergy - management
Type 1 - avoidance if possible, pharmacotherapy, immunotherapy
Type IV - avoidance only
Skin prick testing vs patch testing - indication
SPT - history suggestive of IgE-mediated allergy
Patch testing - History suggestive of contact dermatitis
SPT vs PT - test format
SPT - allergen extract drops applied to skin, skin punctured, read after 15 mins
Patch testing - Test antigen applied under occlusive dressing, read after 48 hrs
SPT vs PT - positive
SPT - wheal and flare response
PT - eczematous reaction
Tuberculin skin test (TST)
- Used to determine exposure to TB
- Chemoprophylaxis may be indicated to reduce risk of reactivation
- Tuberculin injected intradermally (tuberculin = complex mixture of antigens derived from MTB)
- Local inflammatory response evolves over 24-72 hrs if previously exposed
- Fairly poor test for active TB
Mechanism of TST
- Antigen is injected into subcutaneous tissue and processed by local antigen-presenting cells
- A Th1 effector cell recognises antigen and releases cytokines which act on vascular endothelium
- Recruitment of phagocytes and plasma to site of antigen injection causes visible lesion
Over 24-72 hrs
Detection of TB-specific Th1 cells in vitro by interferon gamma release assay (IGRA)
MTP peptides added to blood in lab (ESAT-6, CFP-10) –> antigen presenting cell presents peptides with MHC2 and secretes IL-12
Previous TB exposure
- Effector memory Th1 cells recognise antigen
- Because this is a secondary immune response, they are ‘primed’ and release cytokines within this short timeframe
Th1 memory –> IFN-gamma
No previous TB exposure
No primed effector memory T cells specific for MTB
No interferon gamma produced in such a short timeframe
Th0 (naive) –> Th1
IGRA: positive test by two methods
ELISPOT method (T-SPOT)
ELISA method (quantiferon gold)
ELISA method - controls
Negative control - no interferon gamma
Positive control - high interferon gamma
TB antigen - high interferon gamma
