The antigen binding site of an antibody is present at

Options

A. the constant region
B. the C- terminal
C. the variable region
D. between constant and variable region.

Explanation

**Step-by-Step Solution:**

1. **Understanding Antibody Structure**: Antibodies have a Y-shaped structure composed of four polypeptide chains: two heavy chains and two light chains.

2. **Identifying Regions of Antibodies**: These chains are divided into two main regions: the variable region and the constant region. The variable region is responsible for the specificity of the antibody, while the constant region maintains the structure and function of the antibody.

3. **Locating the Antigen Binding Site**: The antigen binding site, which is crucial for the antibody's function, is located in the variable region. This region varies between different antibodies, allowing them to bind to various antigens.

4. **Evaluating the Options**:
- **Constant Region**: Incorrect, as the antigen binding site is not present here.
- **C Terminal**: Incorrect, as this does not specifically refer to the antigen binding site.
- **Variable Region**: Correct, as this is where the antigen binding site is located.
- **Between Constant and Variable Region**: Incorrect, as the antigen binding site is specifically in the variable region.

5. **Conclusion**: The correct answer to the question is that the antigen binding site of an antibody is present at the **variable region**.

**Final Answer**: The antigen binding site of an antibody is present at the **variable region**.

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