Antigen.

Explanation

### Step-by-Step Text Solution for the Definition of Antigen:

1. **Definition of Antigen**: An antigen is defined as a substance that triggers the immune system to produce antibodies.

2. **Function of Antigen**: When an antigen enters the body, it stimulates the immune response, leading to the production of antibodies that specifically target that antigen.

3. **Nature of Antigens**: Antigens are typically foreign substances to the human body. This means they are not naturally found in the body and are recognized as threats.

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