Antibodies are secreted by

Options

A. T-lymphocytes
B. B-lymphocytes
C. both (a) and (b)
D. natural killer cell

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks about the cells responsible for secreting antibodies.

2. **Identifying the Cell Types**: The options provided include:
- T lymphocytes
- B lymphocytes
- Both A and B
- Natural killer cells

3. **Examining T Lymphocytes**:
- T lymphocytes (T cells) are part of the adaptive immune response.
- They include T helper cells and T cytotoxic cells.
- T helper cells assist B cells in producing antibodies but do not secrete antibodies themselves.

4. **Examining B Lymphocytes**:
- B lymphocytes (B cells) are also part of the adaptive immune response.
- Their primary role is to mediate the antibody-mediated immune response.
- When B cells differentiate into plasma cells, they are responsible for secreting antibodies.

5. **Examining Natural Killer Cells**:
- Natural killer (NK) cells are part of the innate immune response.
- They respond to viral infections and tumors but do not secrete antibodies.

6. **Conclusion**:
- Since T lymphocytes do not secrete antibodies, and natural killer cells do not either, the only correct answer is B lymphocytes, specifically the differentiated B cells (plasma cells) that secrete antibodies.

7. **Final Answer**: The correct answer is **B lymphocytes**.

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