What are antigens?

Explanation

**Step-by-Step Solution:**

1. **Definition of Antigens**: Antigens are substances that can trigger an immune response in the body. They are recognized as foreign materials by the immune system.

2. **Nature of Antigens**: Antigens can be composed of various types of molecules, including proteins, lipids, carbohydrates, or other complex structures.

3. **Examples of Antigens**: Common examples of antigens include:
- Pollen grains
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