HYPERSENSITIVITY REACTION
Exaggerated immune response of the sensitised host which
leads to excessive reactivity and adverse effects
Classification of Hypersensitivity Reactions:
1. Immediate Type:
Timing: Occurs within seconds to minutes after antigen exposure.
Mediated by: Humoral antibodies (B-cell mediated).
Includes: Types 1, 11, III.
2. Delayed Type:
Timing: Develops within 24-48 hours.
Mediated by: T-cell (cell-mediated immunity).
Includes: Type IV.
Types
Type I: Anaphylactic (Atopic) Reaction
Mediated by: IgE antibodies.
Pathogenesis:
During first contact of the host with the antigen
↓
Antigen is captured by antigen presenting cells
↓
Presenting to T cell which differentiates into TH2 cell
↓
TH2 cell releases IL-3, IL-4 and IL-5
↓
IL-4 causes activation of B cell
↓
Activated B cells differentiate to form IgE-secreting plasma cells
↓
IgE antibodies formed
↓
Bind to the Fc receptors present on mast cells
↓
Mast cell are now fully sensitised for the next event
↓
During the second contact with the same antigen (shocking dose)
Ig antibodies on the surface of mast cells-basophils are activated
↓
Causes cell damage-membrane lysis, influx of sodium water
Degraulation of mast cells - basophils
Examples:
• Systemic Anaphylaxis (e.g., bee stings, penicillin).
• Local Reactions (e.g., hay fever, asthma, food allergies, skin rashes).
Clinical Features: Itching, erczema, bronchospasm, anaphylactic shock and.urticaria
