Immunosuppression occurs naturally. The body has innate regulators that have immuno-suppressive effects. However, a deliberate immunosuppression is still necessary, such as to prevent rejection of organ transplants.
After organ transplantation, the body usually detects and considers the newly transplanted tissue as non-self or foreign body and may be harmful to the host, hence, the immune system will attempt to remove and attack the tissue resulting in the death of the tissue. To prevent this, an immunosuppressive agent is usually applied to induce deliberate immunosuppression.
Word origin: From immuno- + suppression.