Which of the following best explains the difference between epitope and an antigen

Options

A. An epitope is any foreign substance , an antigen is a foreign protein
B. An epitope is part of a antigen where an antibody or lymphocyte receptor binds
C. An antigen is the part of an epitope where an antibody or lymphocyte receptor binds
D. Antigen are recognised by B-cells and antibodies , epitopes are recognised by T-cells

Answer

To understand the difference between an epitope and an antigen, we can break down the concepts step by step:

### Step-by-Step Solution:

1. **Define Antigen**:
- An antigen is a larger molecule, often a protein, that is recognized by the immune system. It can be a part of pathogens like bacteria and viruses or any foreign substance that can provoke an immune response.

2. **Define Epitope**:
- An epitope, also known as an antigenic determinant, is a specific part of the antigen. It is the precise region on the antigen that is recognized and bound by antibodies or lymphocyte receptors (such as T-cell receptors).

3. **Relationship Between Antigen and Epitope**:
- The epitope is a subset of the antigen. This means that while all epitopes are parts of antigens, not all parts of an antigen are epitopes. The immune system targets these epitopes to mount a response.

4. **Recognition by Immune Cells**:
- Antigens are recognized by antibodies produced by B cells, while epitopes can be recognized by both B cells (through antibodies) and T cells (through T-cell receptors).

5. **Evaluate the Options**:
- Option 1 states that an epitope is any foreign substance and an antigen is a foreign protein. This is incorrect because epitopes are specific parts of antigens.
- Option 2 states that an epitope is a part of an antigen where an antibody or lymphocyte receptor binds. This is correct.
- Option 3 states that an antigen is the part of an epitope where an antibody or lymphocyte receptor binds. This is incorrect because it reverses the relationship.
- Option 4 states that antigens are recognized by B cells and antibodies, and epitopes are recognized by T cells. This is misleading because epitopes can also be recognized by B cells.

6. **Conclusion**:
- The best explanation of the difference between an epitope and an antigen is given in Option 2: "An epitope is a part of an antigen where an antibody or lymphocyte receptor binds."

### Final Answer:
The correct option is **Option 2**: An epitope is a part of an antigen where an antibody or lymphocyte receptor binds.