Drugs to reduce the risk of rejection are given to organ transplant patients because patients because the donated organ contains?

The answer is; cell surface receptors that are recognized as antigens by the recipient's immune system.

Therefore the organ is attacked as a foreign substance . This is why organ transplants are given by the closest family member to reduce the antigen variability that causes transplant rejection.  Tissue typing is done to ensure that the organ or tissue is as similar as possible to the tissues of the recipient.