Allergy

Allergy

·        = Immunologic reaction to common substances which are harmless to most people

·        Previous exposure ® antibodies or specific lymphocytes against these substances

·        Types:

o   Atopy:

·        Predisposition to produce IgE antibodies to common environmental substances (also called immediate or Type 1 hypersensitivity).

·        Order of incidence:

§  Adults aged 20 – 44 in New Zealand: Asthma 15%, hay-fever 35%, Maori more symptomatic

§  Mediators lead to vasodilation, vascular leakage (swelling), smooth muscle spasm (eg respiratory).

§  Similar symptoms can occur from non-allergic hypersensitivity => non-atopic

o   Contact Allergies: direct skin contact with nickel, chrome, rubber. Due to lymphocyte (delayed-type hypersensitivity, type IV) not IgE antibodies

o   Allergic Alveolitis ® lung inflammation. Eg farmer‟s lung, pigeon fancier‟s lung. Due to lymphocytes and IgG (not IgE)

·        Risk factors:

o   Allergy predominates in young adults and children: while non-specific hypersensitivity is more common later in life

o   Genetic Factors: One parent ® doubled risk of child having atopic disease. Both parents ® 4 times risk

o   Early childhood factors important in subsequent development of allergic disease:

§  High house dust mite/cat/pollen exposure in early months ® ­risk

§  Exposure to tobacco smoke in utero/infancy ® ­risk 

§  Early life infections ® ¯risk: ?improved shift from TH2 environment of uterus to non-allergic TH1 immune responses which dominate in most infections (especially intracellular pathogens) 

§  First born children at greater risk

o   The workplace is a major source of allergen exposure

·        Bee sting allergy:

o   Don‟t have to have atopic history

o   If anaphylaxis as a child, 1 in 6 chance next time.  For adult, 60% chance next time 

o   Carry adrenaline until desensitisation (serial antigen shots ® 95% effective) 

o   Anaphylaxis: give 0.5 m of 1:1000 adrenaline IM if in community setting (iv in hospital if you can give slow infusion). IM gives good diffusion, safer, effective and fewer problems with cardiac vasoconstriction cf bolus