Rheumatoid arthritis is an autoimmune disease.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Definition of Rheumatoid Arthritis**: Rheumatoid arthritis (RA) is a chronic inflammatory disorder that primarily affects joints but can also have systemic effects on the body.

2. **Identifying the Nature of the Disease**: RA is classified as an autoimmune disease. This means that the body's immune system mistakenly attacks its own tissues.

3. **Explaining the Mechanism**: In rheumatoid arthritis, the immune system fails to distinguish between self-cells (the body’s own cells) and non-self-cells (foreign invaders). As a result, it mistakenly identifies the body’s own cells as foreign.

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