Name an allergen and write the response of the human body when exposed to it.

Explanation

### Step-by-Step Solution:

1. **Identify an Allergen**:
- An allergen is any substance that can cause an allergic reaction. A common example of an allergen is **pollen grains**.

2. **Understand the Immune Response**:
- When the human body is exposed to an allergen like pollen, the immune system mistakenly identifies it as a harmful substance, even though it is generally harmless.

...