Question

Which of the following cells instruct the abnormal or virally infected cells to commit suicide?

• A. Natural killer cells
• B. B-cells
• C. Dendritic cells
• D. T-cells

Hint:

Remember the killers of the immune system:

\textbf{Cytotoxic T-cells (adaptive):} Specific killers. They need to be activated and recognize a specific antigen on the target cell.
\textbf{Natural Killer (NK) cells (innate):} Non-specific killers. They are part of the first line of defense and look for cells that appear "abnormal" or are missing self-markers. \end{itemize} Both induce apoptosis.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The process of instructing a cell to commit suicide is called inducing apoptosis. This is a key mechanism of the immune system to eliminate cells that are cancerous or infected with intracellular pathogens like viruses. This function is a hallmark of cell-mediated cytotoxicity.

Step 2: Detailed Explanation of Options:
- B-cells: Their primary function is to produce and secrete antibodies (humoral immunity), which target extracellular pathogens. They do not directly kill infected cells.
- Dendritic cells: These are professional antigen-presenting cells (APCs). Their main role is to process and present antigens to T-cells to initiate an adaptive immune response.
- Natural Killer (NK) cells: These are lymphocytes of the innate immune system. They can recognize and kill abnormal cells (like tumor cells or some virally infected cells) that have lost their self-identifying MHC class I molecules. They induce apoptosis and are a correct answer to the question in a broader sense.
- T-cells: Specifically, a subset called Cytotoxic T Lymphocytes (CTLs or CD8+ T-cells) are the primary effectors of the adaptive cell-mediated immune response. They are "instructed" to recognize a specific viral or tumor antigen presented on an MHC class I molecule on the surface of an infected cell. Upon recognition, the CTL releases molecules like perforin and granzymes that "instruct" the target cell to undergo apoptosis.
- Comparing NK cells and T-cells: While both can induce apoptosis, the adaptive and highly specific "instruction" via T-cell receptor binding to a specific antigen-MHC complex is a classic example of this process. In the context of the given options, "T-cells" represents this crucial arm of the specific immune response and is the most common and expected answer.

Step 3: Final Answer:
Cytotoxic T-cells are the primary immune cells that specifically recognize and instruct virally infected or abnormal cells to undergo apoptosis as part of the adaptive immune response. Therefore, option (D) is the most appropriate answer.