Hypersensitivity Flashcards
Define Hypersensitivity
An inappropriate and excessive immunological reaction to an external allergen due to a dysfunctional control of the immune system
Difference between allergy and anaphylaxis?
Allergy is local response, anaphylaxis is systemic response so involves shock/death
How are hypersensitivity reactions classified?
Types I-IV based on mediators Type I = immediate, by IgE/mast cells Type II = antibody dependent, by IgG/IgM Type III = immune complex, by IC Type IV = delayed/cell-mediated, by T lymphocytes
How do Type I and Type IV reactions differ?
Type IV is slower, specific to antigen and more localised
The brief pathology of Type I reactions?
B cell stimulation, IgE production and binding to mast cells, mast cells release mediators (vasoactive amines, lipid mediators and cytokines)
Initial exposure to antigen causes sensitisation: IgE production and binding to mast cells
What happens following repeat exposure of antigen in Type I reactions?
IgE coated resting mast cell becomes immediately activated from antigen binding and undergoes degranulation to release mediators, transcription of cytokine genes and enzymatic modification of arachidonic acid
What’s an example of a Type IV reaction?
Allergic contact dermatitis - slow, specific, cytotoxic reaction
What blood markers would be raised in Type I reaction?
Tryptase
Eosinophils
IgE
Outline skin prick test - what’s it used for?
To determine exact allergen causing response
Hypodermic needle to punch holes in skin, place drop of diluted antigen with positive (histamine) and negative (saline) controls, wait 15 mins for lesion >3mm
What treatments are there for hypersensitivity reactions?
Steroids Anti-histamines Mast cell stabilisers Leuktriene receptor antagonists Avoid!! Desensitisation therapy
Examples of lipid mediators and their effects?
Prostaglandins and leukotrienes
Vascular dilation and smooth muscle constriction
What are clinical features of Type I reactions?
Airway and eye mucous membranes: pruritus, swelling, rhinorrhoea, lacrimation
Skin: pruritus and urticaria
Oral mucous membranes: angioedema
What mediators are released from mast cell degranulation?
Vasoactive amines -> vascular dilation and smooth muscle contraction
Proteases -> tissue damage
What cell response is responsible for type IV hypersensitivity reactions?
Cytotoxic T lymphocytes
