Understanding allergies Flashcards
Define allergies
Allergy refers to a range of symptoms due to inappropriate activation of the immune system to benign substances such as grass pollen, house dust mite, and foods.
What are the different ways the immune system can kill pathogens?
- Phagocytosis
- Disrupt the cell membrane (complement proteins)
- Release of chemicals
- Killing the cell the pathogen has infected (cytotoxic T cells and NK cells)
Recall the role of mast cells
Mast cells play a crucial role in host defense against parasites:
- Location beneath epithelial surfaces (dermis, GIT, RT) enables rapid response to a range of triggers
- Release of inflammatory mediators
- Recruitment of a range of effector cells
- Increased vascular permeability
- Smooth muscle contraction (remove gastrointestinal contents more rapidly)
List the various substances released by mast cells
- Granule release triggered by binding of antibody to Fc receptors specific for IgE
- Preformed substances
- Histamine (vasodilatation, increased capillary permeability, chemokinesis, bronchoconstriction)
- Heparin (anticoagulant)
- Tryptase (activates C3)
- Eosinophilic chemotactic factor (eosinophil chemotaxis)
- Platelet-activating factor (mediator release)
- Synthesis and release of lipid mediators
- Leukotrienes B4, C4, and D4 (vasoactive, bronchoconstriction, chemotaxis)
- Prostaglandins (affect bronchial muscle, platelet aggregation, vasodilatation)
- Secretion of cytokines (e.g., TNF)
List the four types of hypersensitivity reactions
Hypersensitivity reactions categorized into types:
- Type I – Immediate reactions due to IgE antibodies
- Type II - Cytotoxic effects due to IgM and IgG antibodies
- Type III – Immune complex reactions/serum sickness
- Type IV – Cellular immune mechanisms
Discuss IgE production
Recall CD4 T helper subtypes
Th1 cell releases IFNy
Th2 and IL4
Th17 and IL17
TFH and IL21
Tref
Signals 1, 2, and 3 are given during interaction of naive CD4 Ts and activated DCs.
IL12 and IFNy give ruse to Th1.
IL4 give rise to Th2
Th2 skewing resylts in reease of IL3 5 and 13. This causes class switch recombination to IgE antibody recombination
IgE antibodies circulate, attached to FCER waitign for antigen.
On contact with allergen, degranulation occurs. This is an allergic reaction
What are the consequences of IgE maediated mast cell degranulation?
local systemic
- Local (urticaria and angioedema) or systemic (anaphylaxis)
Describe allergy testingand interpret a positive result
Allergy testing is done to detect IgE antibodies:
- The presence of IgE antibodies to a particular substance is known as sensitization.
- A positive test for IgE antibodies does not necessarily mean that you are allergic.
- Some people have positive testing for IgE antibodies but do not have allergic symptoms.
- The presence of sensitization and the amount of IgE antibody to a particular substance merely means that you have the prerequisites for an allergic reaction.
What are the two methods to detect IgE?
Currently, we detect IgE antibodies by two different methods:
1. Skin prick testing
2. Blood tests for specific IgE
Describe skin prick testing
- Detects localized mast cell degranulation in the skin: if a patient is sensitised, IgE will already be attached to mast cell
- Affected by antihistamines and other medications with antihistamine effects
- Cannot be read accurately in patients with dermatographism
- Risk of reaction to application of allergen
- Requires cooperation
Describe blood test
- Provides an alternative to skin prick testing
- May be more sensitive but less specific compared to skin prick testing
- Does not show that the IgE is capable of degranulating a mast cell
- Blood tests are not affected by medications
Describe atopy
Atopy refers to a predisposition to making IgE antibodies to common environmental proteins such as grass pollens, dust mites, foods, etc. It is:
- Familial, with a number of gene clusters associated
- Can be modified by environmental factors
- Associated with conditions such as eczema, asthma, and food allergies
