Overview
This lecture explains type IV hypersensitivity reactions, distinguishing their mechanisms, immune cells involved, and clinical examples compared to other types.
Hypersensitivity Reactions Overview
- Hypersensitivity reactions are excessive immune responses to harmless substances, causing inflammation, tissue damage, and cell death.
- There are four types of hypersensitivity disorders, each with distinct mechanisms.
Type IV (Delayed/Cell-Mediated) Hypersensitivity
- Type IV hypersensitivity reactions differ by being delayed (days to years), unlike the immediate responses of other types.
- They involve T cells (not antibodies), earning the name "cell-mediated" hypersensitivity.
- Severity of type IV reactions ranges from mild to severe.
Subtypes of Type IV Reactions
- Delayed reactions typically involve only T helper (Th) cells.
- Cytotoxic reactions involve both T helper cells and cytotoxic T cells.
Delayed Reactions
- Usually affect the skin after antigen exposure (e.g., latex, nickel, cosmetics).
- Antigen-presenting cells (e.g., dendritic cells) engulf the antigen and present it to T helper cells in lymph nodes.
- On re-exposure, T helper cells release cytokines, causing local inflammation, dermatitis, rashes, and itchiness.
Cytotoxic Reactions
- Antigen-presenting cells activate T helper and cytotoxic T cells.
- Cytotoxic T cells destroy cells displaying the antigen, which may be cancerous or virally infected (beneficial) or normal tissue (harmful).
- Autoimmune examples:
- Myelin destruction leads to multiple sclerosis (MS).
- Beta cell destruction in the pancreas leads to type 1 diabetes.
Key Terms & Definitions
- Hypersensitivity — excessive immune response to non-harmful substances.
- Type IV hypersensitivity — delayed, T cell-mediated immune reaction.
- T helper cells — immune cells that coordinate immune responses by releasing cytokines.
- Cytotoxic T cells — immune cells that destroy antigen-presenting cells.
- Cytokines — signaling proteins that mediate inflammation.
- Antigen-presenting cells (APCs) — cells that process and present antigens to T cells.
Action Items / Next Steps
- Review the mechanisms distinguishing type IV from other hypersensitivity types.
- Memorize clinical examples like contact dermatitis, multiple sclerosis, and type 1 diabetes.