Introduction
The immune system is designed to protect the body from harmful invaders, but sometimes it overreacts to harmless substances or causes tissue damage while trying to defend the body. These overreactions are known as hypersensitivity reactions. Type IV hypersensitivity, also known as delayed-type hypersensitivity (DTH), is a special kind of immune reaction that occurs 24 to 72 hours after exposure to an antigen. This answer provides a clear and simple overview of Type IV hypersensitivity.
What is Type IV Hypersensitivity?
Type IV hypersensitivity is a cell-mediated immune response rather than an antibody-mediated one. It is mainly caused by T-cells, especially CD4+ helper T-cells and CD8+ cytotoxic T-cells. Unlike the other types (I, II, III), which involve antibodies, Type IV reactions are slower and do not involve antibodies.
Mechanism of Type IV Hypersensitivity
The reaction develops in two phases:
1. Sensitization Phase
- Occurs when a person is first exposed to an antigen (allergen).
- The antigen is taken up by antigen-presenting cells (APCs), like dendritic cells.
- The APCs present the antigen to naïve CD4+ T-cells in lymph nodes.
- These T-cells become sensitized and turn into memory T-cells.
2. Elicitation Phase (Re-exposure)
- When the same antigen enters the body again, the memory T-cells recognize it.
- They release cytokines like IFN-γ (interferon-gamma), TNF-α, and others.
- These cytokines attract macrophages and other immune cells to the site.
- The result is inflammation and tissue damage at the site of exposure.
- The reaction usually becomes visible after 24–72 hours.
Examples of Type IV Hypersensitivity
- Contact Dermatitis: Skin rash from exposure to substances like nickel, poison ivy, or cosmetics.
- Tuberculin Skin Test (Mantoux Test): Used to test exposure to tuberculosis (TB). Swelling at the injection site after 48–72 hours indicates a positive result.
- Granulomatous Inflammation: Chronic inflammation as seen in diseases like tuberculosis and sarcoidosis where immune cells form clusters (granulomas).
- Graft Rejection: Some types of organ rejection are also mediated by T-cells and considered Type IV reactions.
Key Features
| Feature | Description |
|---|---|
| Reaction Type | Cell-mediated (T-cells) |
| Onset Time | Delayed (24–72 hours) |
| Major Cells Involved | CD4+ T-cells, macrophages |
| Antibodies | Not involved |
| Examples | Contact dermatitis, TB test |
Clinical Importance
- Helps diagnose exposure to certain infections (like TB).
- Explains the cause of allergic skin reactions.
- Important in organ transplantation and autoimmune conditions.
Conclusion
Type IV hypersensitivity is a delayed immune response that is mediated by T-cells, not antibodies. It plays a vital role in defending the body but can also lead to harmful effects like allergic skin reactions or chronic inflammation. Understanding this type of hypersensitivity helps in managing allergic reactions, diagnosing infections, and improving transplant outcomes.