Anti-thymocyte globulin (ATG) is a potent medication used to suppress the body’s immune system. It is derived from antibodies, typically from horses or rabbits, that have been immunized with human thymocytes or T-cells. ATG reduces the body’s immune response, which is beneficial in medical situations where an overactive immune system causes harm.
Why ATG Immunosuppression is Used
ATG immunosuppression is employed in various medical conditions where tempering the immune system is beneficial. A primary use is in organ transplantation, such as kidney, heart, and liver transplants, where it helps prevent and treat organ rejection. The immune system identifies the transplanted organ as foreign, leading to rejection. ATG reduces the number of immune cells that might target the new organ, thereby helping the body accept the graft.
ATG is also used to treat severe aplastic anemia, a bone marrow failure disorder where the immune system attacks and damages the body’s own bone marrow stem cells. By suppressing the immune system, ATG allows the bone marrow to recover and produce healthy blood cells. When combined with other medications like cyclosporine, ATG improves blood counts in most aplastic anemia cases, often stopping the need for blood transfusions within three months.
Beyond transplantation and aplastic anemia, ATG finds occasional use in other severe autoimmune diseases. This occurs when conventional treatments have not been effective in controlling the immune system’s attack on healthy body tissues.
How ATG Affects the Immune System
ATG primarily targets and depletes T-lymphocytes, which are central to immune responses and organ rejection. These antibodies bind to markers on the surface of T-cells, leading to their destruction. This targeted depletion reduces the number of T-cells circulating in the body and residing in lymphoid organs.
By reducing the number and activity of T-cells, ATG helps to modulate the immune system. This modulation prevents the immune system from attacking transplanted organs or healthy body tissues in autoimmune conditions. While ATG generally depletes T-cells, some studies suggest it may preferentially deplete naive T-cells while sparing or even increasing the proportion of regulatory T-cells, which play a role in maintaining immune tolerance.
Receiving ATG Treatment
ATG is administered intravenously (IV). This infusion typically takes several hours and is usually performed in a hospital setting. A course of treatment commonly lasts for several days, depending on the specific medical condition being treated and the type of ATG used.
Before each ATG infusion, patients often receive pre-medications to minimize potential infusion-related reactions. These pre-medications can include corticosteroids, antihistamines, and fever reducers. This practice helps to reduce symptoms like fever, chills, or rash that can occur during the infusion.
Close medical supervision is maintained throughout the infusion period. Healthcare professionals monitor vital signs such as temperature, blood pressure, respiratory rate, and heart rate. They also watch for any signs of allergic reactions or other adverse events. Blood tests are regularly performed to check blood cell counts and organ function, allowing for dose adjustments if necessary.
Common Side Effects and Precautions
Patients receiving ATG may experience infusion-related reactions, particularly during the first dose. These common reactions can include fever, chills, skin rash, and changes in blood pressure. Other possible symptoms during infusion include nausea, vomiting, headache, muscle aches, or difficulty breathing. These reactions are typically managed with pre-medications and by adjusting the infusion rate.
A significant concern with ATG is the increased risk of infection due to its immunosuppressive effects. Patients become more susceptible to bacterial, viral, and fungal infections. To mitigate this risk, prophylactic medications may be prescribed, and strict hygiene practices are emphasized. Close monitoring for signs of infection, such as fever or unusual symptoms, is also necessary.
ATG can also affect blood counts. Common effects include leukopenia (a decrease in white blood cells), thrombocytopenia (a low platelet count, which can increase the risk of bleeding), and anemia (a low red blood cell count). Regular blood tests are performed to monitor these counts, and transfusions may be given if levels become too low.
Additional potential effects of ATG can include gastrointestinal upset, muscle and joint pain, and a condition known as serum sickness, which can manifest as rash, fever, and joint pain, typically developing 7 to 21 days after treatment begins. Close medical follow-up, including regular blood tests and symptom monitoring, is performed to manage these side effects and adjust treatment as needed to ensure patient safety.