Allergy Flashcards
How does an allergy develop?
- Phase 1 - Induction Phase
- Phase 2 - Reactive phase
What is the induction phase? (sensitisation)
- Allergen captured by antigen-presenting cells
- Presents part of allergen (epitope) to T cells
- Specific T cells (Th2 Lymphocytes) produce IL-4, IL5 and IL-13
- Stimulate B cells to produce IgE antibodies
- IgE antibodies bind to basophils and mast cells
What is the reactive phase?
Subsequent exposure to allergen:
* Cross-linking of antibodies
* Degranulation
* Release of histamine
* Also, leukotrienes, protaglandins, chemotactic
What are the consequences of degranulation?
- Vasodilation/Oedema—-> Hives
- Bronchoconstriction—->Asthma
- Mucus Production—->Rhintis
What are common allergens?
- House dust mites
- Pollen
- Animal fur
- Molds
- Food
How do you identify allergies?
- Skin prick test
- Liquid allergen pricked into skin
- Weal and flare forms if allergic
What happens when it goes wrong?
- E.g. Allergic asthma
- Loss of epithelium
- Bronchoconstriction
- Increase mucus
- Oedema
- Remodelling
What is the role of Histamine?
H1 Receptors:
* Contraction of smooth muscle (ileum, airways, uterus)
* Dilation of blood vessels
* Inc permeability of capilliaries
* Sensory nerves–stimulate itching
* CNS–arousal/wakefulness (antagonists cause sedation)
H2 Receptors:
* Inv in acid secretion in stomach
* Inc HR
H3 Receptors:
* Pre-synaptic—Inhibit release of Neurotransmitters incl histamine
H4 Receptors:
* Possibly role in inflammation
What are Histamine receptor antagonists?
- H1 receptor antagonists:
1. Allergy
2. Sedation
3. Nausea & Vomiting (e.g. Travel sickness) - H2 Receptor antagonists:
1. Reduce acid secretion e.g. treatment of peptic ulcers
What is allergic Rhinitis?
- Perennial–House dust mite & animal hair/fur
- Seasonal–Most common=hay fever
- Association with atopy (Gen)
- Occupational e.g. Latex
- Nasal itching
- Rhinorrhea
- Sneezing
- Conjunctival symptoms–(worsened by irritants, itchy red eye)
What is the Pathophysiology of allergic rhinitis
- Mast cell degranulation
- Histamine is 1o mediator from mast cells & basophils
1. Acts on H1 receptors
2. H2 & H3 a bit - Nasal obstruction - Also role of leukotrienes + PGs
How do you manage allergic rhinitis?
Avoidance:
* Pollen in spring, stay indoors
1. Glasses
* Dust Mite
1. Vacuum house
2. Wooden floors vs carpets
3. Wash bed clothes at 60oC
4. Freeze pillows
What are antihistamines? (Histamine H1 recep antagonists)
H1 antagonists remove rhinorrhoea, itching, conjunctivitis & sneezing but not nasal obstruction
Sedating:
* Chlorphenamine (Piriton)
* Alimenazine (Trimeprazine)
* Promethazine (Phenergan)
- Penetrate BBB– act on H1 receptors in the brain
- Not ideal, effects of alcohol enhanced
What are non-sedating antihistamines?
- Acrivastine (Benadryl)
- Cetrizine (Zirtek)
- Fexofenadine (Telfast)
- Loratadine (Clarityn)
- Dont cross BBB
What are topical antihistamines?
- Nasal-For rhinitis
- Eye drops in allergic conjuctivitis
- Rapid relief, but localised effect
What are corticosteroids?
- Intanasal e.g. Beclometasone (Beconase)
- Anti-inflammatory
1. Reduce cytokines and chemokines thus reduce recruitment of immune cells in epithelial mucosa.
What is the difference of antihistamines and steroids?
What are Cromones?
E.g. Sodium cromoglicate & nedocromil sodium
- Nasal and eye
- Mast cell stabiliser–Inhinits mast cell degranulation
- Eye drops–Most effective for conjuctival symptoms
What is Nasal Congestion?
- Blocked or stuffy nose
- E.g. Allergic rhinitis or common cold
- Excess mucus production
- Histamine causes inc blood flow leads to nasal tissue swelling (oedema)
What are Decongestants?
Symptomatic relief–Reduce feeling of congestion
Topical (nasal):
* a Adrenoreceptor agonists. Act on a adrenoreceptors to cause vasoconstriction
* Reverse vasodilation–prevents tissue oedema
* Reduce flow mucus
* Act locally
* Rebound congestion
What are systemic decongestants?
- Phenylephrine a1 adrenoreceptor-selective
Indirectly acting–Releasing NA:
* Pseudoephedrine e.g. Sudafed
* Less effective than topical decongestants
What is Anaphylaxis?
- Severe allergic reaction
- Bronchoconstriction, tight throat
- Vasodilation leads to drop in Bp
- Itchiness
- Puffy face, swollen tongue
How do you treat analphylaxis?
- Oxygen
- Fluid-Inc BP
- Adrenaline–Inc cardiac output & bronchodilation–EpiPen
- B2-adrenoceptor agonist (salbutamol)
- Histamine H1 receptor antagonist i.m. or iv
- Hydrocortisone (steroid) i.v.
Summaries the treatments of allergies?
- Histamine H1 receptor antagonists mainstay of treatment of allergy
- Prevent effects of histamine
- Steroid for more serious allergy
- Anaphylaxis–Life threatening allergic reaction
