Question

Adaptive Immunity: Second Line of Defense—Cellular Defenses

The T-cell receptors and CD4 and CD8 molecules interact with major histocompatibility complex (MHC) proteins. MHC proteins are found on most cells of the body. There are two variations, or classes, of MHC proteins.

MHC class I is expressed on the surface of most cells of the body such as skin cells, liver cells, and others. Should a cell become infected with a virus, viral antigens are processed in the cells and expressed on the cell surface within the MHC class I molecule. Cytotoxic T-cells, expressing their T-cell receptor and CD8 molecule, can recognize this viral particle within the MHC class I and, if the cytotoxic T-cell has been activated by IL-2, the cytotoxic T-cell will react by releasing granzymes and perforin. The release of granzymes and perforin has the effect of killing the virally-infected cell and preventing the spread of infection further.

MHC Class II is expressed only on the surface of antigen-presenting cells such as macrophages. When macrophages phagocytose pathogens or antigens from the extracellular fluid, the macrophages display the antigens on the surface of their cells using MHC Class II. A helper T-cell that expresses its T-cell receptor and CD4 molecule can recognize this antigen. The helper T-cell will react by releasing IL-2 into the extracellular fluid. This IL-2 can activate both the helper T-cell as well as nearby cytotoxic T-cells. The helper T-cell will proliferate in response.

Read the report and answer all the questions.

Kidney transplant can be required when a patient has sustained extensive damage to a kidney through injury or disease. When a patient needs a kidney transplant, the team of transplant doctors searches for an available kidney that expresses a very similar major histocompatibility complex (MHC) to the host patient. MHC matching is essential to the long-term health of the transplanted kidney. Once the kidney is transplanted, the kidney cells interact with the cells of the immune system. If the MHC molecules expressed on the transplanted kidney cells are too different from the host MHC molecules, the immune cells can confuse them with MHC molecules carrying foreign antigens. An immune reaction can result in which the cells of the transplanted kidney die by apoptosis. This process, called transplant rejection, will eventually compromise the transplanted kidney and a new kidney will need to be found.

1. Which MHC class molecule is most critical to match for transplant success?

• MHC Class II

• MHC Class I

2. Which immune cells directly mediate the apoptosis of the kidney cells and therefore the rejection of the kidney itself?

• Dendritic cells

• Helper T-cells

• Cytotoxic T-cells

3. Would a drug that inhibits the secretion of IL-2 be helpful to prevent transplant rejection?

• Yes, IL-2 inhibitor drugs would be useful in that they would prevent the inflammatory response.

• No, IL-2 inhibitor drugs would accelerate the immune system’s rejection of the kidney.

• Yes, IL-2 inhibitor drugs would be useful in that they would prevent the activation of cytotoxic T-cells.

• No, IL-2 inhibitor drugs would have no effect on transplant rejection.

Answer 1