American Board of Surgery Qualifying Exam (ABS QE) Practice Test

What is the immunosuppressive effect of Anti-thymocyte globulin (ATG) primarily attributed to?

1. Depletion of CD8+ lymphocytes

2. Activation of macrophages

3. Acting on T-cell surface antigens

4. Blocking cytokine receptors

The correct answer is: Acting on T-cell surface antigens

The immunosuppressive effect of Anti-thymocyte globulin (ATG) is primarily attributed to its action on T-cell surface antigens. ATG is a polyclonal antibody preparation derived from the serum of immunized rabbits or horses, specifically targeting human T-lymphocyte antigens. When ATG is administered, it binds to various surface markers on T cells, leading to the destruction of these cells through complement-mediated lysis and opsonization. This results in a significant reduction in the overall T cell population, particularly affecting the activation and proliferation of T cells, which are crucial for initiating and sustaining immune responses. By targeting the surface antigens present on T cells, ATG effectively induces a state of immunosuppression. This mechanism is particularly important in clinical situations like organ transplantation, where it helps to prevent acute rejection by inhibiting the activation of T cells that would otherwise attack the transplanted organ. Other options, while related to immune processes, do not accurately describe the primary mechanism of action for ATG. For instance, depletion of specific lymphocyte subtypes or the blocking of cytokine receptors involves different mechanisms that are not the main focus of ATG's activity. Thus, the effectiveness of ATG as