Type Iv Hypersensitivity

Type Iv Hypersensitivity

Introduction

• Cell-Mediated) Reactions/Delayed-type hypersensitivity/antibody-independent
• Involve reactions by memory cells.
• First contact sensitizes person.
• Subsequent contacts elicit a reaction.
• Reactions are delayed by one or more days (delayed type hypersensitivity).
• Delay is due to migration of macrophages and T cells to site of foreign antigens.
• Reactions are frequently displayed on the skin:  itching, redness, swelling, pain.
• Tuberculosis skin test
• Poison ivy
• Metals
• Latex in gloves and condoms (3% of health care workers)

Peak action time

• after 24 hours
• Anaphylactic shock may occur 

Initiated by

•  activated T-lymphocytes.

 Includes

• Delayed type hypersensitivity – mediated by CD-4 T cells.
• T-cell mediated cytotoxicity – mediated by CD8- T cells.

Delayed type hypersensitivity

• CD-4 T cells and macrophages are primarily involved
• Mechanism of defence against various intracellular pathogens
• mycobacteria
• fungi
• certain parasites.
• Pathophysiology
• Characterized by formation of granuloma
• a localized mass that contains activated macrophages (epitheloid) cells 
• surrounded by collar of lymphocytes and giant cells.
• It is also called granulomatous inflammation.
• Antigen presenting cell comes in contact with naive CD-4 T cells
• Naive CD-4 T cells recognize these peptide antigens in association with MHC – II and differentiate in to T4-1 cells.
• Expression of delayed hypersensitivity largely depends on the cytokines secreted by T4-1 cells.

 T-cell mediated cytotoxicity

• In this type of hypersensitivity, cytotoxic T cells kill virus infected or tumor cells.

Type IV hypersensitivity Test

• Injection is given intradermally on the flexor aspect of the forearm.
• The site is examined after 48-72 hours.
• Induration of greater than 10 mm is considered as positive.
• Examples → Tuberculin test, Lepromin test