Why is tissue incompatibility an obstacle in organ transplantation?

Organ transplants are very diverse. Almost all important organs are transplanted. This can be skin, kidneys, part or whole liver, spinal cord, heart, and so on. But it is not always possible to transplant a donor organ to a patient. It may not be compatible with the patient’s tissue. If such an organ, bone marrow or skin is transplanted, the rejection of a new part of the body will begin. The body will not understand what it is and will begin to defend itself. It also happens that an organ that is suitable in all parameters begins to be rejected by the body.