All are the true statements about IgM except :-

Options

A. Pentamer form
B. Can cross placenta
C. Synthesis after acute infection in blood
D. Mostly intravascular in distribution

Answer

To solve the question "All are the true statements about IgM except:", we will analyze each statement about Immunoglobulin M (IgM) and identify the incorrect one.

### Step-by-Step Solution:

1. **Understanding IgM**:
- IgM is an immunoglobulin, which is a type of antibody produced by vertebrates. It is known for being the largest antibody in terms of size.

2. **Identifying the Characteristics of IgM**:
- **Pentameric Structure**: IgM exists in a pentameric form, meaning it is composed of five subunits (monomers) linked together. This is a true statement.
- **Crossing the Placenta**: IgM does not cross the placenta from the mother to the fetus. This is a false statement and is the one we are looking for.
- **Synthesis After Acute Infection**: IgM is typically synthesized in response to an acute infection, making this statement true.
- **Distribution**: IgM is primarily found in the intravascular compartment, which is also a true statement.

3. **Evaluating the Options**:
- Option A (Pentameric form): True
- Option B (Can cross placenta): False (this is the incorrect statement)
- Option C (Synthesis after acute infection): True
- Option D (Mostly intravascular in distribution): True

4. **Conclusion**:
- The statement that is not true about IgM is that it can cross the placenta. Therefore, the answer to the question is **Option B**.

### Final Answer:
The incorrect statement about IgM is that it can cross the placenta.

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