In thymic T cell development, positive selection ensures thymocytes can recognize self-MHC. Which statement best describes positive selection?

In thymic T cell development, positive selection ensures thymocytes can recognize self-MHC. Which statement best describes positive selection?

• A. Positive selection eliminates thymocytes that fail to recognize self-MHC in the cortex.
• B. Positive selection eliminates thymocytes that recognize self-antigens too strongly in the thymus.
• C. Positive selection promotes B cell maturation in bone marrow.
• D. Positive selection drives clonal expansion of mature T cells in the spleen.

Answer: (A)

Positive selection in the thymus tests whether thymocytes’ TCRs can recognize self-MHC with enough affinity to receive survival signals, and this happens in the cortex. Thymocytes that can interact with self-MHC survive and proceed to become CD4 or CD8 cells, while those that fail to recognize self-MHC die by neglect. This step ensures that mature T cells are MHC-restricted. The statement describing positive selection as eliminating thymocytes that fail to recognize self-MHC in the cortex fits this concept precisely. The other ideas refer to negative selection (eliminating cells that recognize self-antigens too strongly) or to processes outside the thymus (B cell maturation in bone marrow or peripheral expansion of mature T cells in the spleen), which are not what positive selection describes.

Positive selection in the thymus tests whether thymocytes’ TCRs can recognize self-MHC with enough affinity to receive survival signals, and this happens in the cortex. Thymocytes that can interact with self-MHC survive and proceed to become CD4 or CD8 cells, while those that fail to recognize self-MHC die by neglect. This step ensures that mature T cells are MHC-restricted. The statement describing positive selection as eliminating thymocytes that fail to recognize self-MHC in the cortex fits this concept precisely. The other ideas refer to negative selection (eliminating cells that recognize self-antigens too strongly) or to processes outside the thymus (B cell maturation in bone marrow or peripheral expansion of mature T cells in the spleen), which are not what positive selection describes.