Allergy

Question 1

How does an allergy develop?

Answer 1

1. Phase 1 - Induction Phase
2. Phase 2 - Reactive phase

Question 2

What is the induction phase? (sensitisation)

Answer 2

• 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

Question 3

What is the reactive phase?

Answer 3

Subsequent exposure to allergen:
* Cross-linking of antibodies
* Degranulation
* Release of histamine
* Also, leukotrienes, protaglandins, chemotactic

Question 4

What are the consequences of degranulation?

Answer 4

1. Vasodilation/Oedema—-> Hives
2. Bronchoconstriction—->Asthma
3. Mucus Production—->Rhintis

Question 5

What are common allergens?

Answer 5

• House dust mites
• Pollen
• Animal fur
• Molds
• Food

Question 6

How do you identify allergies?

Answer 6

• Skin prick test
• Liquid allergen pricked into skin
• Weal and flare forms if allergic

Question 7

What happens when it goes wrong?

Answer 7

• E.g. Allergic asthma
• Loss of epithelium
• Bronchoconstriction
• Increase mucus
• Oedema
• Remodelling

Question 8

What is the role of Histamine?

Answer 8

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

Question 9

What are Histamine receptor antagonists?

Answer 9

• 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

Question 10

What is allergic Rhinitis?

Answer 10

• 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)

Question 11

What is the Pathophysiology of allergic rhinitis

Answer 11

• 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

Question 12

How do you manage allergic rhinitis?

Answer 12

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

Question 13

What are antihistamines? (Histamine H1 recep antagonists)

Answer 13

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

Question 14

What are non-sedating antihistamines?

Answer 14

• Acrivastine (Benadryl)
• Cetrizine (Zirtek)
• Fexofenadine (Telfast)
• Loratadine (Clarityn)
• Dont cross BBB

Question 15

What are topical antihistamines?

Answer 15

• Nasal-For rhinitis
• Eye drops in allergic conjuctivitis
• Rapid relief, but localised effect

Question 16

What are corticosteroids?

Answer 16

• Intanasal e.g. Beclometasone (Beconase)
• Anti-inflammatory
  1. Reduce cytokines and chemokines thus reduce recruitment of immune cells in epithelial mucosa.

Question 17

What is the difference of antihistamines and steroids?

Answer 17

Question 18

What are Cromones?

Answer 18

E.g. Sodium cromoglicate & nedocromil sodium

• Nasal and eye
• Mast cell stabiliser–Inhinits mast cell degranulation
• Eye drops–Most effective for conjuctival symptoms

Question 19

What is Nasal Congestion?

Answer 19

• 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)

Question 20

What are Decongestants?

Answer 20

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

Question 21

What are systemic decongestants?

Answer 21

• Phenylephrine a1 adrenoreceptor-selective

Indirectly acting–Releasing NA:
* Pseudoephedrine e.g. Sudafed
* Less effective than topical decongestants

Question 22

What is Anaphylaxis?

Answer 22

• Severe allergic reaction
• Bronchoconstriction, tight throat
• Vasodilation leads to drop in Bp
• Itchiness
• Puffy face, swollen tongue

Question 23

How do you treat analphylaxis?

Answer 23

• 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.

Question 24

Summaries the treatments of allergies?

Answer 24

• Histamine H1 receptor antagonists mainstay of treatment of allergy
• Prevent effects of histamine
• Steroid for more serious allergy
• Anaphylaxis–Life threatening allergic reaction