Antigen binding site of immunoglobulin is

Options

A. variable region of heavy chain
B. variable region of light chain
C. constant region of light chain
D. variable region of both heavy and light chain

Answer

To answer the question about the antigen binding site of immunoglobulin, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Structure of Immunoglobulin**:
- Immunoglobulins (Ig) are Y-shaped molecules made up of two types of polypeptide chains: heavy chains and light chains.

2. **Identify the Regions of Immunoglobulin**:
- Each immunoglobulin has two main regions: the variable region and the constant region.
- The constant region is similar across different immunoglobulins, while the variable region varies among different antibodies.

3. **Locate the Antigen Binding Site**:
- The antigen binding site is present in the variable regions of the immunoglobulin.
- This site is crucial as it is where the antibody binds to the specific antigen.

4. **Analyze the Options**:
- **Variable region of heavy chain**: This is partially correct, but it does not encompass the entire binding site.
- **Variable region of light chain**: This is also partially correct for the light chain but misses the heavy chain's contribution.
- **Constant region of light chain**: Incorrect, as the constant region does not participate in antigen binding.
- **Variable region of both heavy and light chains**: This is the correct answer because both regions contribute to the antigen binding site.

5. **Conclusion**:
- The antigen binding site of an immunoglobulin is located in the variable regions of both the heavy and light chains.

### Final Answer:
The antigen binding site of immunoglobulin is in the **variable region of both heavy and light chains**.

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