S1i / 15A15 : Describe the different types of hypersensitivity reactions including an example of each

15A15: Exam Report

Describe the different types of hypersensitivity reactions including an example of each

79% of candidates passed this question.

This question can be answered in tabular form and details have been described in a previous exam report (2007). Details of each of the four main types were expected. A description of the timing of reactions was also expected.

S1i / 15A15: Describe the different types of hypersensitivity reactions including an example of each

Definition

Hypersensitivity – refers to an abnormal reaction produced by a normal immune system

Classification

Type

Timing

Mechanism

Example

Type

Type I Anaphylactic

Timing

Immediate; mins-hrs

Mechanism

IgE antibodies on surface of mast cells bind to foreign Ag causing mass MC degranulation.  Releases into circulation:  histamines, serotonin, leukotrientes, platelet activating factor and heparin.  Resulting in VD, increased cap perm, mucous secretion and sm m spasm.  <1hr following exposure

Example

Suxamethonium Anaphylaxis

Type

Type II  Antibody-dependant

Timing

Variable

Mechanism

IgE & IgM bind to Ag and activate MF, NK and classical complement pathway → results in target cell lysis

Example

Blood transfusion reactions

Type

Type III Immune-complex mediated

Timing

1-3 weeks post exposure

Mechanism

Ag-Ab complexes in plasma deposit in vessels and tissues and activate the classical complement pathway

Example

SLE

Type

Type IV  Delayed

Timing

48-72hrs

Mechanism

Cell mediated, does not involve Ab/complement.  T cells previously sensitised to Ag become activated on re-exposure and damage the cell

Example

Contact dermatitis

Anaphylaxis v Anaphylactoid

Anaphylaxis

Prior exposure

Anaphylactoid

No prior exposure

No reaction on first Ag exposure

Reaction can occur on first exposure

IgE mediated

Not IgE mediated

Severe reaction may be fatal

Reaction less severe

Reaction not related to dose of Ag

Severity is related to dose of agent

Eg Anaphylaxis to SUX

Eg reaction to IV contrast