Immune response to first encounter with an antigen is called:

Options

A. Primary immune response
B. Secondary immune response
C. Memory
D. Clonal selection

Answer

**Step-by-Step Solution:**

1. **Understanding Immune Response**: The immune response is the body's defense mechanism against foreign substances, known as antigens. When the body encounters an antigen for the first time, it triggers a specific immune response.

2. **Identifying the Types of Immune Responses**: There are two main types of immune responses:
- **Primary Immune Response**: This occurs upon the first exposure to an antigen. The immune system recognizes the antigen and begins to produce specific antibodies and activate T-cells.
- **Secondary Immune Response**: This occurs upon subsequent exposures to the same antigen. The immune system responds more rapidly and effectively due to the presence of memory cells.

3. **Analyzing the Options**: The question provides four options:
- Primary Immune Response
- Secondary Immune Response
- Memory
- Clonal Selection

4. **Evaluating the Options**:
- **Primary Immune Response**: This is the correct answer as it specifically refers to the immune response during the first encounter with an antigen.
- **Secondary Immune Response**: This is incorrect as it refers to the response during subsequent encounters with the same antigen.
- **Memory**: This term refers to the immune system's ability to remember past antigens, but it does not specifically answer the question.
- **Clonal Selection**: This is a theory related to how B cells respond to antigens, but it does not define the first encounter with an antigen.

5. **Conclusion**: Based on the analysis, the immune response to the first encounter with an antigen is called the **Primary Immune Response**.

**Final Answer**: Primary Immune Response

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