Antigen is

Options

A. Substances which stimulates the production of venom
B. Vaccine
C. Antibody production stimulating agent
D. Part of the body defense system

Answer

### Step-by-Step Solution:

1. **Definition of Antigen**: An antigen is a substance that triggers an immune response in the body. It is recognized by the immune system as a foreign entity.

2. **Origin of the Term**: The term "antigen" is derived from the words "antibody" and "gen," where "gen" refers to the formation or production of something.

3. **Role of Antigens**: When a pathogen (like bacteria or viruses) enters the body, it has specific molecules on its surface known as antigens. These antigens are recognized by the immune system.

4. **Immune Response**: Upon recognizing the antigens, the immune system activates B cells, which are a type of white blood cell. These B cells then differentiate into plasma cells.

5. **Antibody Production**: The plasma cells produce antibodies in response to the antigens. There are five main classes of antibodies (immunoglobulins): IgG, IgA, IgM, IgE, and IgD.

6. **Correct Answer**: The correct characterization of an antigen is that it is an "antibody production stimulating agent." This means that antigens stimulate the immune system to produce antibodies.

7. **Misconceptions**: It is important to clarify that antigens are not vaccines or parts of the body’s defense system; they are external agents that provoke an immune response.

### Final Answer:
Antigen is an antibody production stimulating agent.

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