A substance produced by the host in response to an infection of foreign structure is

Options

A. Antigen
B. Phytotoxin
C. Antibody
D. Hormone

Answer

To answer the question "A substance produced by the host in response to an infection of foreign structure is," we will analyze the options provided: antigen, phytotoxins, antibody, and hormones.

### Step-by-Step Solution:

1. **Understanding Antigens**:
- Antigens are foreign substances that enter the body and trigger an immune response. They are not produced by the host but are recognized by the immune system as invaders.
- **Hint**: Remember that antigens are the invaders, not the substances produced by the host.

2. **Understanding Phytotoxins**:
- Phytotoxins are toxic compounds produced by plants. They are not related to the human immune response and are not produced by the host.
- **Hint**: Phytotoxins are plant-derived and not relevant to human infections.

3. **Understanding Hormones**:
- Hormones are chemical messengers produced by glands in the body. They regulate various physiological processes but are not specifically produced in response to infections.
- **Hint**: Hormones are involved in regulation, not in direct immune response to infections.

4. **Understanding Antibodies**:
- Antibodies are specific proteins produced by B cells in response to the presence of antigens. They are designed to bind to antigens and help neutralize or eliminate them.
- Antibodies are crucial components of the immune response and are specifically produced when the body detects foreign structures.
- **Hint**: Antibodies are the body's defense mechanism against infections, produced specifically in response to antigens.

5. **Conclusion**:
- Based on the analysis of the options, the correct answer is **antibody**. Antibodies are the substances produced by the host (specifically B cells) in response to foreign antigens.

### Final Answer:
**Antibody**