Which of the following is not a difference between IgG and IgM?

Options

A. a.IgM is a pentamer while IgG is a monomer.
B. b.IgG is formed early during the primary response, IgM is formed later.
C. c.IgG can cross the placenta, IgM usually does not.
D. d.IgG has two antigen binding sites, IgM has ten.

Answer

To solve the question "Which of the following is not a difference between IgG and IgM?", we will follow these steps:

### Step 1: Understand the characteristics of IgG and IgM
- **IgM (Immunoglobulin M)**:
- It is a pentamer (five units linked together).
- It has a J-shaped polypeptide chain.
- It appears during the early stages of infection.
- It cannot cross the placenta.
- It has 10 antigen-binding sites.

- **IgG (Immunoglobulin G)**:
- It is a monomer (single unit).
- It has a Y-shaped polypeptide chain.
- It appears in the late stages of infection.
- It can cross the placenta.
- It has 2 antigen-binding sites.

### Step 2: Analyze the options
Now we will analyze the provided options to identify which statement is not a difference between IgG and IgM.

1. **Option A**: IgM is a pentamer while IgG is a monomer.
- This is true and is a difference.

2. **Option B**: IgG is formed during the primary response and IgM is formed later.
- This is false. IgM is formed during the primary response, and IgG is formed later. Hence, this is not a difference.

3. **Option C**: IgG antibodies can cross the placenta and IgM usually doesn't cross the placenta.
- This is true and is a difference.

4. **Option D**: IgG has 2 antigen-binding sites whereas IgM has 10.
- This is true and is a difference.

### Step 3: Conclusion
Based on the analysis, **Option B** is not a difference between IgG and IgM antibodies.

### Final Answer
**Option B** is the correct answer as it is not a difference between IgG and IgM.

---