Pediatric Liver Transplantation Information for Healthcare Professionals

Chapter 8: Immune System

The immune system is a complex network of specialized organs and cells which produce chemicals that have the ability to recognize foreign substances as they enter the body.  It has three different functions: defensive, homeostasis and surveillance.  The defense function eliminates bacteria and viruses.  The homeostatic function removes damaged or dead cells from the body and maintains the body’s internal environment.  And finally, the surveillance function removes abnormal cells.  The immune system is able to distinguish between self and nonself.  An antigen is a substance, from a virus, bacteria, fungus, blood or an organ, that is recognized as a foreign substance by the immune system.  When the body identifies a “nonself,” it produces antibodies to fight the foreign substance or object.  The antibodies remember previous experiences and react accordingly.  For example, once your child has had chicken pox, the antibodies in his immune system will prevent him from getting the virus again.

The success of transplantation depends on the body’s ability to accept the new organ.  This is accomplished by suppressing the body’s natural ability to get rid of a foreign object.  Two different mechanisms are used to prevent rejection of the organ.  First, prior to transplantation, tissues of the donor and recipient are matched (tissue typing or histocompatibility testing).  If the donor and recipient share at least two or more antigens, they are likely to be a “good” match.  A second approach in preventing rejection is altering the immune system’s response through powerful immunosuppressant medications.  The medications your child will take are used to control or stop the body’s natural response against a foreign substance and to prevent episodes of rejection.  Finding the right dose for your child is a balancing act between the conflicting needs of preventing rejection and of avoiding infection.  The goal of immunosuppressive therapy, drugs such as Cyclosporine, Prograf, Prednisone, Cellcept and Imuran is to intercept the message sent between the specialized cells that make up the immune system: the B cells, T cells and the lymphokines.  B cells produce antibodies that have a specific target.  T cells are responsible for rejection of transplanted tissue even when the donor and recipient are a close match.  Lymphokines are powerful substances released from activated immune cells which signal or recruit other immune cells to join the battle against foreign invasion.

Surface antigens from the transplanted organ are released and are recognized by the recipient’s immune system as a foreign substance or unmatched tissue.  When the body is exposed to a foreign substance or antigen, a defensive response is mobilized quickly and efficiently against the particular invader.  In other words, the T cells and B cells send messages to the tissue and cells and the signal is interpreted with the help of the lymphokines.  Cell surface receptors, substances which recognize antigens on living cells, also play an active role in defending the body by recognizing viruses and bacteria.