The most important long acting antibody representing about `80%` of the antibody that is able to pass across the placenta is:

Options

A. IgD
B. IgG
C. IgM
D. IgA

Answer

To solve the question regarding the most important long-acting antibody that represents about 80% of the antibodies able to pass across the placenta, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Types of Antibodies**:
- There are several classes of antibodies (immunoglobulins), including IgD, IgG, IgM, and IgE. Each has different functions and characteristics.

2. **Identify the Function of Each Antibody**:
- **IgD**: Primarily found on the surface of B cells and acts as an antigen receptor. It constitutes about 0.2% of total antibodies.
- **IgG**: This is the most abundant antibody, making up about 80% of the total antibodies in the blood. It is known for its ability to cross the placenta and provide immunity to the fetus.
- **IgM**: This antibody constitutes about 5-10% of total antibodies and is the largest antibody. It does not cross the placenta.
- **IgA**: This antibody accounts for about 10-15% of total antibodies and is found in secretions like saliva, tears, and breast milk. It also does not cross the placenta.

3. **Focus on the Key Characteristics**:
- The question specifically asks for the antibody that is long-acting and capable of passing through the placenta.

4. **Determine the Correct Answer**:
- Given that IgG is the only class of antibody that can cross the placenta and represents about 80% of the total antibodies, it is the correct answer.

5. **Conclusion**:
- Therefore, the most important long-acting antibody that is able to pass across the placenta is **IgG**.

### Final Answer:
The most important long-acting antibody representing about 80% of the antibody that is able to pass across the placenta is **IgG**.