Hypersensitivity Flashcards
what is hypersensitivity?
a state of altered reactivity in which the body reacts with an exaggerated immune response to a foreign agent
what is an allergen?
an antigen that causes an allergic reaction
what is the Gell and Coombs’s classification?
the 4 types of hypersensitivity
what is the most common and rapid hypersensitivity?
type 1
what is type 1 hypersensitivity in response to?
allergen (antigen) so sometimes called immediate hypersensitivity
what is type 1 hypersensitivity mediated by?
IgE so sometimes called IgE mediated hypersensitivity
what is atopy?
a genetic predisposition for allergy
what is anaphylaxis?
acute serious allergic response
what are the 2 types of allergens
proteins and haptens
what is a hapten?
any small molecule that can be recognised by a specific antibody but cannot elicit an immune response
how do haptens elicit an immune response?
they must be chemically linked to a protein molecule (carrier) to elicit antibody and/or T-cell responses
what does a protein allergen have the ability to do?
elicit an immune response
what is the mechanism of type 1 hypersensitivity?
IgE producing B cells activated during sensitisation, IgE binds to Fc receptor on mast cells or CD63 on basophils (APCs), IgE recognises allergen and next exposure binds rapidly and causes immediate degranulation resulting in allergy symptoms
what is the main role of hypersensitivity 1
immune cell recruitment, neurotransmitter, vasodilator, endothelial constriction, bronchoconstrictor
what are the examples of type 1 hypersensitivity?
cutaneous atopy, systemic anaphylaxis
what does cutaneous atopy cover?
allergic rhinitis (hay fever), atopic dermatitis (allergic eczema), asthma (lower respiratory tract)
what is systemic anaphylaxis?
anaphylactic shock throughout the body
how does systemic anaphylaxis arise?
from mast cell degranulation
what are anaphylatoxins produced by?
the complement cascade
what are the ways to test for allergies?
skin prick test and patch test, blood test, food challenge
what is a skin prick test and patch test used for?
a skin reaction and are done to identify suspected allergen
what do blood tests measure?
IgE levels in blood
what does a food challenge do?
diagnoses food allergy
what are the different anti-allergy methods?
avoid allergen, drugs, immunological treatment
what drugs are used for anti-allergy purposes?
anti-histamines, hydrocortison, cromoglycate, epinephrine
what does anti-histamines do?
compete with histamine for receptors and can be inverse agonists or antagonists
what does hydrocortisone do?
block histamine synthesis
what does cromoglycate do?
stabilises mast cells and stops histamine release
what does epinephrine do?
stimulation of adrenoreceptors (for anaphylactic shock), increases peripheral vascular resistance, improves blood pressure, reverses peripheral vasodilation, causes bronchodilation, reduces inflammatory mediator release
what types of immunological treatment are available for anti-allergy purposes?
hypo- or de- sensitisation (repeat injections of allergen) and IgE to IgG production
what type of response is atpoy?
localised
what type of response is anaphylaxis?
systemic
what does type 2 hypersensitivity involve
activation of complement by IgG or IgM binding to an antigenic cell (cell surface auto-antigens or haptens/allergens)
in type 2 hypersensitivity how are the cells destroyed?
either by membrane attack complex formation (complement) or by antibody dependent cell mediated cytotoxicity (natural killer cells or CD8+ T cells)
give a common example of hypersensitivity 2
transfusion reactions/autoimmune hemolytic anaemia (mismatched blood type)
what does type 3 hypersensitivity involve?
reactions against soluble antigens circulating in serum
what happens in type 3 hypersensitivity?
antibody-antigen immune complexes are deposited in organs which leads to complement activation, neutrophil recruitment, inflammation-mediated damage
what are the examples of type 3 hypersensitivity?
Arthus reaction, serum sickness, oral erythema multiforme
what is arthus reaction and what does it arise from?
localised hypersensitivity reaction, antibody-antigen complexes are localised e.g. blood vessel walls and can arise from injected particles (vaccinations) or from inhaled or ingested allergens (farmers lung and coeliac disease)
what is serum sickness?
a systemic form of arthus reaction, antibody-antigen complexes circulate and lodge in many different tissues
what is oral erythema multiforme characterised by?
crusty blistering of oral mucosa
what is oral EM caused by?
deposition of immune complexes (IgM-bound immune complexes) in the microvasculature of the oral mucous membrane
what type of inflammatory response is oral EM?
can be an acute inflammatory response to viral infections e.g. HSV
what drug can cause oral EM?
trimethoprimsulfamethoxazole which is widely administered to treat urinary or lower respiratory tract infections
what is type 4 hypersensitivity
T-cell mediated hypersensitivity (no antibodies)
why is type 4 hypersensitivity sometimes called delayed hypersensitivity
it takes time to recruit T cells (adaptive immune cells)
what occurs in type 4 hypersensitivity?
localised T cell reaction at site of antigen exposure e.g. contact dermatitis
what cells does type 4 hypersensitivity involve?
CD4+ (inflammation induced damage via macrophages) and CD8+ (direct damage of cells -cytotoxicity) T cells
what are the examples of type 4 hypersensitivity?
contact dermatitis, tissue graft rejection, OFG, response to intracellular pathogens e.g. TB characterised by the formation of granulomas in lung tissue
what mediates type 1 hypersensitivity
IgE (mast cells and antibodies)
what mediates type 2 hypersensitivity?
IgG or IgM leading to complement activation and/or antibody dependent cell mediate cytotoxicity
what mediates type 3 hypersensitivity?
antibody-allergen
what mediates type 4 hypersensitivity?
T cells (no antibodies required)
