Of the following about antibodies and antigens , choose the correct set
I. An antibody consisits of four identical light (L) chains and two identical heavy (H) chains
II. The stem of Y of antibody is called Fab fragment.
III. The stem of antibody and lower parts of its arms constitute constant (C) region
III. Portion of antigen to which an antibody binds is called epitope

Options

A. II and IV
B. I and III
C. III and IV
D. I and II

Answer

To determine the correct set of statements regarding antibodies and antigens, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement I: An antibody consists of four identical light (L) chains and two identical heavy (H) chains.**
- This statement is incorrect. An antibody is composed of two identical heavy chains and two identical light chains, not four identical light chains.

2. **Statement II: The stem of Y of antibody is called Fab fragment.**
- This statement is also incorrect. The stem of the Y-shaped antibody is known as the Fc fragment, while the arms of the Y, which contain the antigen-binding sites, are referred to as the Fab (Fragment antigen-binding) fragments.

3. **Statement III: The stem of antibody and lower parts of its arms constitute constant (C) region.**
- This statement is correct. The constant region of the antibody includes the stem (Fc region) and the lower parts of the arms, which do not vary between different antibodies of the same class.

4. **Statement IV: Portion of antigen to which an antibody binds is called epitope.**
- This statement is also correct. The specific part of the antigen that is recognized and bound by an antibody is known as the epitope.

### Conclusion:
From the analysis:
- Statement I: Incorrect
- Statement II: Incorrect
- Statement III: Correct
- Statement IV: Correct

Thus, the correct set of statements about antibodies and antigens includes **Statements III and IV**.

### Final Answer:
The correct set of statements is: **III and IV**.

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