Natural killer (NK) cells can destroy

Options

A. Invading microorganisms
B. Virus infected cells
C. Some tumor cells
D. All of these

Answer

### Step-by-Step Solution:

1. **Understanding Natural Killer (NK) Cells**:
- NK cells are a type of lymphocyte that play a crucial role in the innate immune system, which is the body's first line of defense against pathogens. They are known for their ability to recognize and destroy compromised cells.

2. **Function of NK Cells**:
- NK cells are cytotoxic, meaning they can kill other cells. They are activated by proteins called interferons, which signal the immune system to respond to infections.

3. **Targets of NK Cells**:
- **Invading Microorganisms**: NK cells can target and kill invading microorganisms, which include bacteria and other pathogens.
- **Virus-Infected Cells**: They are particularly effective against cells that have been infected by viruses, helping to prevent the spread of the virus within the body.
- **Tumor Cells**: NK cells also play a role in detecting and controlling tumor cells, making them important in cancer surveillance.

4. **Evaluating the Options**:
- Option 1: Invading microorganisms – True.
- Option 2: Virus-infected cells – True.
- Option 3: Tumor cells – True.
- Option 4: All of these – Since all previous options are true, this option is also correct.

5. **Conclusion**:
- Therefore, the correct answer to the question "Natural killer (NK) cells can destroy" is **Option 4: All of these**.