1. Background

• allergy is the manifestation of a hypersensitivity reaction to the presentation of an allergen due to the propensity of the affected individual to develop a sustained IgE response following antigenic stimulation
• the distinguishing feature of the allergic individual is the propensity to develop a sustained IgE response
• there are several types of allergies based upon where the hypersensitive reaction occurs:

2. Allergens

• allergens are compounds capable of inducing human IgE antibody formation ("sensitization")

3. IgE

• IgE is a Homocytotropic Antibody: an antibody capable of interacting with target cells such that these cells release mediators on contact with specific antigens
• the IgE receptor consists of an externally located alpha chain that binds the IgE and is noncovalently associated with a beta chain and two gamma chains
• there are two categories of IgE receptors dependent upon the type of cells targetted and the type of mediators released from these cells:

1. Background

• an allergy specific to the skin in susceptible individuals

2. Allergens

3. IgE

• within the superficial layers of the epidermis of the skin a Type 1 homocytotropic antibody response occurs
• mast cells, basophils are the target cells
• mediator response:
  • release of histamine -> intensely pruritic response associated with hives
  • also stimulates the axon reflex -> wheal and flare reaction

1. Background

• non-immunologic events cause degranulation of mast cells resulting in the release of histamine within the superficial layers of the epidermis of the skin

2. Triggers

1. Urticaria

• erythematous raised skin lesion (wheal)
  • localized or generalized
  • well circumscribed but sometimes coalescent
  • may be intensely pruritic
  • most common over bony prominences
  • vary in size from tiny flat papules to large raised plaques
  • flat centre with raised erythematous edge
  • diagnostic feature is transience of individual lesions
    • small individual lesions usually last from 24-48 hours but new lesions may continue to appear singly or in crops
    • large lesions may last longer but tend to change shape
• types:
  • 1. Acute - urticaria lasts less than 6 weeks
  • 2. Chronic - urticaria lasts greater than 6 weeks

2. Angioedema

• similar skin lesion as urticaria but involves the deeper layers of the dermis and submucosal or subcutaneous tissues
• may also involve:
  • upper airway - with stridor +/- obstruction
  • gastrointestinal tract

1. Benadryl (Diphenhydramine HCl)

• 2.5 cc po q4-6h (< 2 years of age)
• 5.0 cc po q4-6h (2-6 years of age)
• 10-20 cc po q4-6h (> 6 years of age)

2. Atarax (Hydroxyzine HCl)

• drug of choice in chronic and cholinergic urticaria