Type Iii Hypersensitivity

Introduction

• Type III reactions (Arthus reaction etc.) are immune complex diseases
• Peak action time within 6 hours
• Involve reactions against soluble antigens circulating in serum.
• Usually, involve IgA antibodies.
• Large amounts of circulating antigen can form immune complexes:
• which are not easily cleared by phagocytic cells
• Deposited in organs
• Activate complement
• Cause inflammatory damage.

Glomerulonephritis: 

• Inflammatory kidney damage.
• Occurs with slightly high antigen-antibody ratio is present.

Insect bites

• The initial reaction will be type I at the site of the bite 
• 4-8 hours later a type III reaction might develop 

 Arthus reaction:

• Deposits of immune complexes draw neutrophils
• Leading to an accumulation of fluid (edema) and RBC’s (erythema)
• Involves antigen-antibody complexes
• Which precipitate around blood vessels and cell membranes causing damage.
•  Severity of the reaction varies from mild swelling and redness to tissue necrosis
• Cutaneous vasculitis

 Hyperacute graft rejection

• Pre-existing antibody against donor graft antigens can cause
• This form of rejection occurs within minutes or hours after transplantation

 Serum sickness

• following the injection of an anti-toxin

SLE

• Nephritis
• Skin lesions
• Arthritis
• Antinuclear antibody specific for SLE is Anti ds DNA 

Hypersensitivity pneumonitis

• Immunologically mediated response to an extrinsic antigen
• involves
• immune complex (Type III) 
• cell mediated (Type IV) hypersensitivity reactions. 

 Other Examples

• Rheumatoid arthritis
• lnfective endocarditis
• PAN
• Henoch-Schonlein purpura

Type III hypersensitivity Test

• Toxin is given intradermally and this toxin reacts with circulating antitoxin
• Produce local type of type III hypersensitivity, i.e. Arthus reaction.
• Example → Schick test.