What is autoimmune disease? Give an example.

Explanation

### Step-by-Step Solution:

1. **Understanding Immunity**:
- Immunity is the body's ability to resist and fight off disease-causing microorganisms, known as pathogens. This includes bacteria, viruses, and other harmful entities.

2. **Role of Immune Cells**:
- The immune system comprises various cells that help protect the body. Key components include leukocytes (white blood cells), B-lymphocytes, T-lymphocytes, and antibodies. These cells work together to identify and eliminate pathogens.

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