Anti-Thymocyte Globulin, or ATG, is a medication used to suppress the immune system. It is a key component in organ transplantation, such as for the kidney, and in stem cell transplants for conditions like aplastic anemia. The goal of using ATG is to prevent the patient’s body from rejecting the new organ or cells. This medication consists of antibodies that target and neutralize specific cells within the immune system, which temporarily weakens the immune response to help the body accept the transplant.
The Purpose of ATG in Transplantation
The immune system identifies and eliminates foreign invaders using specialized white blood cells called T-lymphocytes, or T-cells. When a person receives a new organ or donor stem cells, their T-cells recognize these new cells as foreign. This recognition triggers an immune attack.
This reaction can lead to two major complications. In solid organ transplants, the recipient’s T-cells attack the new organ, a process called organ rejection. In stem cell transplants, the new, donated immune cells (the graft) attack the recipient’s body, a dangerous condition known as Graft-versus-Host Disease (GVHD).
ATG’s function is to address this by targeting and removing T-cells. The medication consists of antibodies derived from horse or rabbit plasma that has been exposed to human T-cells. These purified antibodies are administered to the patient, where they bind to T-cells and mark them for destruction, significantly reducing their numbers.
This reduction of T-cells provides a window for the transplant to establish itself without being attacked. ATG is used as “induction therapy” to prevent rejection or GVHD from happening. It is also used as a treatment if signs of rejection or GVHD have already appeared.
The Administration Process
The administration of ATG occurs in a hospital setting due to its potency and the potential for infusion-related reactions. Patients are closely monitored throughout the treatment. The medication is delivered intravenously through a central venous catheter, a tube inserted into a large vein, because ATG can be irritating to smaller peripheral veins.
A course of ATG treatment spans several consecutive days. Each daily infusion is given slowly over four to six hours. The exact duration and dosage are tailored to the individual patient, their weight, and the specific reason for the treatment. This slow infusion rate helps to minimize the intensity of potential reactions.
To reduce the risk of adverse reactions, patients receive pre-medications 30 to 60 minutes before each ATG infusion. This protocol includes a corticosteroid to suppress inflammation, an antihistamine to block reactions like rashes, and a fever-reducer like acetaminophen. Medical staff remain attentive during and after the administration, ready to adjust the infusion rate or provide supportive care if any reactions occur.
Potential Side Effects and Management
Receiving ATG can trigger immediate or delayed side effects. The most common issues arise during or shortly after the infusion, caused by the rapid destruction of T-cells. This process releases inflammatory proteins called cytokines, which can lead to symptoms like fever, chills, shaking, a skin rash, and changes in blood pressure.
Medical teams anticipate these reactions and manage them by slowing or pausing the ATG infusion. The pre-medications given before the infusion are the first line of defense in lessening these effects. A more intense version of this reaction, known as Cytokine Release Syndrome (CRS), is less common but requires prompt medical intervention.
A delayed reaction known as Serum Sickness can appear about one to two weeks after treatment. This condition occurs as the patient’s immune system recognizes the ATG antibodies as foreign and forms immune complexes that deposit in tissues. Symptoms include fever, a widespread rash, and joint pain, and the condition is managed with oral corticosteroids.
An expected effect of ATG is a drop in blood cell counts. Because ATG targets T-lymphocytes, white blood cell counts will fall, and platelet counts can also decrease. This requires close monitoring of the patient’s blood work, and platelet transfusions may be required to reduce the risk of bleeding.
Post-Treatment Considerations
Once the course of ATG is complete, the patient enters a prolonged period of immunosuppression. The depletion of T-cells leaves the body with a weakened defense system that can last for several weeks to months. During this vulnerable period, as the bone marrow produces new T-cells, the risk of infection is elevated.
The body’s reduced ability to fight off pathogens means patients are susceptible to infections from viruses, fungi, and bacteria. Organisms that a healthy immune system would control can cause serious illness. Common post-transplant viral infections, such as Cytomegalovirus (CMV) and Epstein-Barr virus (EBV), pose a particular threat.
To counteract this risk, patients are prescribed prophylactic medications. These preventative drugs include antivirals, antibiotics, and antifungals, which are taken for an extended period after ATG therapy. Adherence to this medication schedule is important while the immune system rebuilds.
Frequent follow-up appointments are scheduled to monitor recovery. These visits involve blood tests to track the return of immune cells, allowing the medical team to assess immune reconstitution and watch for signs of infection. Patients are also educated on how to minimize exposure to germs by practicing good hygiene and avoiding crowded places.