Hypersensitivity Reactions | Medical Science Optional Notes for UPSC PDF Download

Introduction

• The protective immune responses can, at times, lead to tissue damage.
• Hypersensitivity encompasses immune reactions causing injury, resulting in hypersensitivity diseases.
• Traditionally, hypersensitivity reactions are categorized into four types, each associated with the principal immune mechanism causing injury. Three involve variations in antibody-mediated injury, while the fourth is T cell-mediated.

Type I Hypersensitivity

Type II Hypersensitivity

Type III Hypersensitivity

Immune Complex Formation

Immune Complex Deposition

Immune Complex-Mediated Inflammation and Tissue Injury

Type IV Hypersensitivity

Gell and Coombs classification of hypersensitivity reactions

Type V/VI Hypersensitivity

Natural Killer Cells

• 10-15% of peripheral lymphocytes
• Lack TCR/Cell surface Ig
• Contain abundant azurophilic granules (Large granular lymphocytes)
• Constituent of the innate immune system (No prior sensitization required)
• CD16/CD56+ (Specific to NK cells)
• Capable of lysing IgG-coated target cells through ADCC
• NK cells possess two types of receptors— inhibitory and activating.
• Inhibitory receptors recognize self class I MHC molecules present on all healthy cells, while activating receptors identify molecules expressed or upregulated on stressed, infected, or DNA-damaged cells.
• Typically, the inhibitory receptors outweigh the activating ones, preventing NK cell activation.

ADCC