A hormone is -

Options

A. An enzyme
B. Chemical messenger
C. Primary messenger
D. 2 and 3 both

Answer

### Step-by-Step Solution:

1. **Understanding Hormones**:
- Hormones are chemical substances produced by endocrine glands. They are released into the bloodstream and act as messengers to regulate various physiological processes in the body.

2. **Types of Hormones**:
- Hormones can be classified into different types based on their chemical nature. The two main categories are:
- **Protein Hormones**: These are made up of amino acids and are water-soluble.
- **Steroid Hormones**: These are derived from cholesterol and are lipid-soluble.

3. **Function of Hormones**:
- Hormones function as **primary messengers**. They bind to specific receptors on target cells to initiate a response.

4. **Secondary Messengers**:
- In some cases, the binding of a hormone (like insulin) to its receptor does not directly cause a response. Instead, it triggers the production of **secondary messengers** inside the cell, which then propagate the signal and lead to a cellular response.

5. **Conclusion**:
- Based on the definitions and functions discussed, a hormone is not an enzyme but a **chemical messenger** that acts as a **primary messenger**. Therefore, the correct answer to the question "A hormone is -" is that it is a **chemical messenger** and a **primary messenger**.

### Final Answer:
A hormone is a **chemical messenger** and a **primary messenger**.

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