What are central tolerance and peripheral tolerance, and how do they prevent autoimmunity?

• A. Central tolerance eliminates self-reactive lymphocytes in thymus/bone marrow; peripheral tolerance includes anergy, Treg suppression, and immune privilege.
• B. Central tolerance uses B cells only; peripheral uses T cells only.
• C. Central tolerance occurs in the spleen; peripheral in lymph nodes.
• D. Central tolerance promotes autoimmunity; peripheral tolerance prevents it.

Answer: (A)

Central tolerance and peripheral tolerance are two layers that keep self-reactive lymphocytes from attacking the body. Central tolerance happens during development in the primary lymphoid organs: T cells in the thymus and B cells in the bone marrow. In this stage, cells with high affinity for self-antigens are deleted or edited, so autoreactive clones are largely removed before they enter circulation. Peripheral tolerance acts on any self-reactive cells that escape central tolerance or arise later, using several controls: T cells can become anergic if they encounter antigen without proper costimulation, regulatory T cells suppress autoreactive responses, and repeated activation can trigger activation-induced cell death. Immune-privileged sites also shield certain tissues from immune attack, reducing exposure to self-antigens. Together, these layers prevent autoimmunity by eliminating or inactivating self-reactive cells at development and restraining their activity in the periphery. The described answer fits this by correctly naming where central tolerance occurs and listing peripheral mechanisms like anergy, regulatory T cell suppression, and immune privilege. The other options misstate which organs or cell types are involved or imply that central tolerance promotes autoimmunity, which it does not.