Which one among the following is not a component of third line of body defence?

Options

A. killer T-cell
B. natural killer cells
C. memory cells
D. plasma cells

Answer

To solve the question "Which one among the following is not a component of the third line of body defense?", we need to understand the components of the immune system, specifically the third line of defense, also known as the adaptive immune system.

### Step-by-Step Solution:

1. **Identify the Components of the Third Line of Defense**:
- The third line of defense is primarily composed of lymphocytes, which include B cells and T cells.
- B cells are responsible for producing antibodies, while T cells (including killer T-cells) are involved in directly attacking infected cells.

2. **List the Given Options**:
- Killer T-cell
- Natural killer cell
- Mammary cells
- Plasma cells

3. **Analyze Each Option**:
- **Killer T-cell**: This is a type of T cell that plays a role in the adaptive immune response by targeting and killing infected cells. It is part of the third line of defense.
- **Natural killer cell**: This is a component of the innate immune system. Natural killer cells do not show specificity towards particular antigens and act more generally against infected or cancerous cells. Therefore, they are not part of the adaptive immune system.
- **Mammary cells**: These cells are involved in the production of milk in mammals and are not related to the immune response.
- **Plasma cells**: These are differentiated B cells that produce antibodies, making them a crucial part of the adaptive immune response.

4. **Determine the Correct Answer**:
- Among the options, the **natural killer cell** is not a component of the third line of defense, as it belongs to the innate immune system.

### Final Answer:
The correct answer is **Natural killer cell**.