Final answer:
Organ rejection is caused by the recipient's immune system recognizing foreign antigens on the transplanted organ, due to differences in MHC molecules. Natural killer cells, B lymphocytes, and T cells attack these foreign proteins, leading to rejection, which necessitates the use of immunosuppressive drugs.
Step-by-step explanation:
An immune system's rejection of organs from an organ donor is primarily caused by the immune system's recognition of foreign antigens. Specifically, proteins on the donated organ, which are typically glycoproteins and glycolipids found on the cell surface, are recognized as foreign due to the distinct MHC (Major Histocompatibility Complex) molecules they present. The MHC molecules, also known as human leukocyte antigens (HLA) in humans, are essential for the immune system to distinguish self from non-self. When an organ with different MHC molecules is transplanted into a recipient, various immune cells, including natural killer cells, B lymphocytes, and T cells, can identify these foreign MHC molecules as non-self. This triggers an immune response that can lead to rejection of the transplanted organ. To minimize this response, transplant recipients are often placed on immunosuppressive drugs to reduce the activity of their immune system and lower the chances of rejection. Additionally, genetic matching and testing for MHC compatibility are critical for increasing the success rate of organ transplants.