Which of the following describes a mechanism used to achieve peripheral tolerance?

• A. Negative selection of autoreactive T cells in the thymus
• B. Apoptosis of autoreactive B cells in the bone marrow
• C. Editing of B-cell receptors that weakly recognize self-antigens in the bone marrow
• D. Lack of a costimulatory signal to autoreactive T cells in the lymph nodes

Answer: (B)

Peripheral tolerance keeps self-reactive lymphocytes from causing autoimmunity after they leave the primary lymphoid organs. A central way this happens is T cell anergy: when a T cell encounters self-antigen in a peripheral site like a lymph node but does not receive the necessary second signal (costimulation), it becomes functionally inactivated and cannot respond later. This lack of costimulation directly prevents autoreactive T cells from mounting an immune response in the periphery.

The other options describe central tolerance mechanisms. Removing autoreactive T cells in the thymus eliminates them before they reach the periphery, and editing or deleting autoreactive B cells in the bone marrow likewise prevents self-reactive B cells from entering circulation. Thus, the described peripheral mechanism is the absence of a costimulatory signal leading to T cell anergy in the lymph nodes.