hypersensitivity Flashcards
what is the definition of hypersensitivity?
state of altered reactivity in which the body reacts with an exaggerated immune response to a foreign agent
what type of reaction is allergy?
hypersensitivity
what is an allergen?
an antigen that causes an allergic reaction
what are some examples of oral allergies?
- gluten
- peanuts
- eggs
etc.
how many types of hypersensitivity are there (Gell and Coombs’s classification)?
4
describe type 1 hypersensitivity
- IgE mediated
- mast cells/basophils
- eg atopy/anaphylaxis
describe type 2 hypersensitvity
- cytotoxic responses
- NK cells/complement/T cells (CD8+ cytotoxic)
- eg haemolytic aneamia
describe type 3 hypersensitivity
- immune complex responses
- complement/neutrophils
- eg serum sickness
describe type 4 hypersensitivity
- cell mediated responses
- T cells/macrophages
- eg sarcoidosis/TB/Crohn’s disease
what is the most common type of hyersensitivity?
type 1
what is the most rapid type of hypersensitivity?
type 1
what is atopy?
genetic predisposition for allergy
what is anaphylaxis?
acute, serious allergic response
what occurs when allergens bind to IgE antibodies?
- degranulation
- reaction
what can allergens be?
proteins or haptens
what is a hapten?
- any small molecule that can be recognised by a specific antibody but cannot elicit an immune response
- must be chemically linked to a protein molecule carrier to elicit an antibody/T cell response
what occurs on first exposure?
IgE producing B cells activated
class switching
what is atopy?
the genetic tendency to develop allergic diseases to allergens
what are examples of cutaneous atopy?
(localised)
- hay fever
- allergic eczema
- lower respiratory tract
what is systemic anaphylaxis?
- anaphylactic shock throughout the body
- arises from mast cell degranulation
- anaphylatoxins can also be produced by a complement cascade (type 2 or 3)
what are methods of allergy testing?
- skin prick test/patch test
- blood test
- food challenge (oral)
describe anti-allergy drugs
- Avoid allergen
- Drugs
Anti-histamines – compete with histamine for receptors
Hydrocortisone – block histamine synthesis
Cromoglycate – stabilises mast cells stops histamine release
Epinephrine – best immediate treatment for anaphylactic shock reverses effects of granules (vasoconstriction, relaxes muscles) - Immunological treatment
Hypo- or de- sensitisation – repeat injections of allergen
IgE to IgG production
how to anti-histamines act?
compete with histamine for receptors
- histamine receptors have basal activity
- can be inverse agonists or antagonists
describe the action of epinephrine
- stimulation of adrenoceptors (anaphylactic shock)
- increases peripheral vascular resistance
- improves blood pressure
- reverses peripheral vasodilation
- causes bronchodilation
- reduces inflammatory mediator release
describe in detail type 2 hypersensitivity
- Involves activation of complement by IgG or IgM binding to an antigenic cell (cell surface auto-antigens or haptens/allergens)
- Cell is lysed by;
Membrane attack complex formation (complement)
Antibody dependent cell mediated cytotoxicity (natural killer cells or CD8+ T cells) - Common example – transfusion reactions/autoimmune hemolytic anaemia
what are examples of type 2 hypersensitivity?
- transfusion of mismatched blood type
- autoimmune hemolytic anaemia
describe in detail type 3 hypersensitivity
- Involves reactions against soluble antigens circulating in serum.
- Antibody-antigen immune complexes are deposited in organs which leads to;
Complement activation
Neutrophil recruitment
Inflammation-mediated damage - Examples include;
Arthus reaction
Serum sickness
Oralerythema multiforme(EM)
what are examples of type 3 hypersensitivity?
- Arthus reaction (localized hypersensitivity reaction)
- Antibody-antigen complexes are localized e.g., blood vessel walls
- Can arise from injected particles (e.g., booster vaccinations) but also can arise from inhaled or ingested allergens (e.g., farmers lung and coeliac disease)
what are examples of type 3 hypersensitivity?
- Serum sickness is a systemic form of Arthus reaction
Antibody-antigen complexes circulate and lodge in many different tissues.
Similar mechanism occurs as localized form. - Oralerythema multiforme(EM)
Characterized by crusty blistering of oral mucosa
Caused by deposition ofimmune complexes(IgM-bound immune complexes) in the microvasculatureof the oral mucous membrane
Can be acute inflammatory responses to viral infections e.g., HSV
Can be caused by hypersensitivity to drugs e.g., Trimethoprim-sulfamethoxazole which is widely administered to treat urinary or lower respiratory tract infections.
describe in detail type 4 hypersensitivity
- T cell-mediated hypersensitivity (no antibodies)
- Sometimes called delayed hypersensitivity as takes time to recruit T cells (adaptive immune response)
- Localized T cell reaction at site of antigen exposure (e.g., contact dermatitis)
- Involves both CD4+ T cells (inflammation induced damage via macrophages) and CD8+ T cells (direct damage of cells – cytotoxicity)
what are examples of type 4 hypersensitivity?
- Contact dermatitis
- Tissue graft rejection
- Response to intracellular pathogens (e.g., TB) – characterized by the formation of granulomas in lung tissue.
- Oral diseases – Orofacial granulomatous (OFG)?
what makes type 4 hypersensitivity different?
no antibodies
