Question

Which of the following is called as cell-mediated (delayed) hypersensitivity:

• A. Type II hypersensitivity
• B. Type I hypersensitivity
• C. Type III hypersensitivity
• D. Type IV hypersensitivity

Hint:

Type IV hypersensitivity involves T-cells and is delayed, unlike the rapid antibody-mediated responses seen in Type I, II, and III hypersensitivities.

Answer 1

The Correct Option is D

Step 1: Understanding hypersensitivity types. Hypersensitivity reactions are classified into four types based on their immune mechanisms:
 - Type I: Immediate hypersensitivity (e.g., allergies, mediated by IgE). 
- Type II: Antibody-mediated cytotoxic hypersensitivity. 
- Type III: Immune complex-mediated hypersensitivity. -
 Type IV: Cell-mediated (delayed) hypersensitivity. 

Step 2: Characteristics of Type IV hypersensitivity. Type IV hypersensitivity is mediated by T-cells rather than antibodies. It is referred to as delayed hypersensitivity because the reaction typically occurs 24–72 hours after exposure to the antigen. 
Examples include: 
- Tuberculin skin test. 
- Contact dermatitis (e.g., poison ivy). 
- Granuloma formation in chronic infections. 

Step 3: Why other options are incorrect. - Type I, II, and III are antibody-mediated hypersensitivities and are not delayed or cell-mediated.

Answer 2

The question asks which hypersensitivity type is described as cell-mediated and delayed. Let's check the four classes against this description.

1. Type II hypersensitivity: This type is driven by antibodies (IgG or IgM) binding directly to antigens on the surface of cells, triggering their destruction. Since antibodies drive it, it is not classified as cell-mediated.
2. Type I hypersensitivity: This is the immediate allergic type, caused by IgE antibodies binding to mast cells and basophils, triggering histamine release within minutes of exposure. Being both antibody-driven and fast, it does not match "cell-mediated" or "delayed."
3. Type III hypersensitivity: This type results from antigen-antibody immune complexes depositing in tissues and triggering inflammation. It is still antibody-mediated, not cell-mediated.
4. Type IV hypersensitivity: This type is driven by sensitized T-cells rather than antibodies. Because T-cells need time to recognize the antigen, become activated, and recruit other immune cells, the reaction typically shows up 24 to 72 hours after exposure, which is why it is called delayed-type hypersensitivity. The tuberculin skin test and poison ivy contact dermatitis are classic examples.

Types I, II, and III all rely on antibodies and act quickly, while Type IV is the one driven by T-cells acting over a delayed timeframe, matching the description in the question.

So the correct answer is Type IV hypersensitivity.