The most characteristic feature of haemoglobin is its ability to combine reversible with ...............

Explanation

**Step-by-Step Text Solution:**

1. **Understanding Hemoglobin's Role**: Hemoglobin is a protein found in red blood cells that plays a crucial role in transporting gases in the body. Its primary function is to carry oxygen from the lungs to the tissues and organs.

2. **Reversible Binding**: One of the most important characteristics of hemoglobin is its ability to bind to oxygen in a reversible manner. This means that hemoglobin can attach to oxygen when it is abundant (such as in the lungs) and release it when it is needed (such as in the tissues).

3. **Oxygen Affinity**: Hemoglobin has a high affinity for oxygen, especially in areas where the concentration of oxygen is high (like the lungs). This allows hemoglobin to effectively pick up oxygen and form a compound known as oxyhemoglobin.

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