The first antibody to appear in the serum following stimulation by antigen is:

Options

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

Answer

To solve the question regarding which antibody is the first to appear in the serum following stimulation by an antigen, we will analyze the options provided and the characteristics of each antibody type.

### Step-by-Step Solution:

1. **Understanding Antibody Types**: There are several classes of antibodies (immunoglobulins) in the human body, including IgG, IgM, IgE, and IgA. Each has different roles and timelines in the immune response.

2. **Identifying the First Antibody**: The question asks for the first antibody that appears in the serum after an antigen is introduced. In the primary immune response, the body initially responds to an antigen by producing antibodies.

3. **Role of IgM**: Immunoglobulin M (IgM) is the first antibody produced by B cells upon initial exposure to an antigen. It is primarily found in the blood and is crucial for the early stages of the immune response.

4. **Comparing Other Antibodies**:
- **IgG**: This antibody appears later in the immune response and is more abundant in the serum during secondary responses.
- **IgE**: This is involved in allergic reactions and does not play a primary role in the initial response to most antigens.
- **IgA**: This antibody is mainly found on mucosal surfaces and also appears after IgM in the immune response.

5. **Conclusion**: Based on the information, the first antibody to appear in the serum following antigen stimulation is **IgM**.

### Final Answer:
The first antibody to appear in the serum following stimulation by an antigen is **IgM**.

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