Which of the following is not a part of second line of defense in human?

Options

A. Neutrophils
B. Macrophages
C. NK Cells
D. B-lymphocytes

Answer

To determine which of the following is not a part of the second line of defense in humans, we need to understand the three lines of defense in the immune system.

### Step-by-Step Solution:

1. **Understand the Immune System's Defense Mechanisms**:
- The human immune system has three lines of defense against pathogens:
- **First Line of Defense**: Physical and chemical barriers (e.g., skin, mucous membranes).
- **Second Line of Defense**: Non-specific immune responses (e.g., white blood cells like neutrophils and macrophages, inflammation).
- **Third Line of Defense**: Specific immune responses (e.g., B-lymphocytes and T-lymphocytes).

2. **Identify the Options**:
- The question provides several options, which likely include various types of white blood cells and possibly other immune components.

3. **Analyze Each Option**:
- **Neutrophils**: These are a type of white blood cell that play a crucial role in the first line of defense by responding quickly to infections.
- **Macrophages**: These are also white blood cells that are part of the second line of defense, as they engulf and digest pathogens.
- **Natural Killer Cells**: These are another type of white blood cell that are part of the second line of defense, targeting infected or cancerous cells.
- **B-lymphocytes**: These are part of the third line of defense, as they are responsible for producing antibodies against specific pathogens.

4. **Determine the Correct Answer**:
- Since neutrophils and B-lymphocytes are not part of the second line of defense, and only macrophages and natural killer cells are, the answer to the question is that B-lymphocytes are not part of the second line of defense.

5. **Conclusion**:
- The correct answer is B-lymphocytes, as they are part of the third line of defense.