Rejection of tissue or organ transplants is brought about mainly by:

Options

A. Suppressor T-cell
B. B-cells
C. Cytotoxic T-cells
D. Memory T-cells

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks about the main cells responsible for the rejection of tissue or organ transplants. This involves understanding the immune response and the different types of cells involved.

2. **Identifying the Cell Types**: The options provided are:
- Suppressor T-cells
- B-cells
- Cytotoxic T-cells
- Memory T-cells

3. **Function of Suppressor T-cells**: Suppressor T-cells help prevent autoimmune diseases by ensuring that the body's immune cells do not attack its own tissues. They are not primarily involved in transplant rejection.

4. **Function of B-cells**: B-cells are responsible for producing antibodies, which target foreign antigens. While they play a role in the immune response, they are not the main cells involved in transplant rejection.

5. **Function of Memory T-cells**: Memory T-cells are involved in remembering past infections and responding more effectively upon subsequent exposures to the same pathogen. They do not directly cause transplant rejection.

6. **Function of Cytotoxic T-cells**: Cytotoxic T-cells, also known as killer T-cells, are crucial in the rejection of transplanted tissues or organs. They identify and destroy foreign cells that are recognized as non-self, which is the primary mechanism behind transplant rejection.

7. **Conclusion**: Based on the functions of the different cell types, the correct answer to the question is that rejection of tissue or organ transplants is mainly brought about by cytotoxic T-cells.

**Final Answer**: Cytotoxic T-cells.

---