In the thymus, positive selection of immature T cells is based upon recognition of which of the following?

• A. MHC antigens
• B. Self-antigens
• C. Stress proteins
• D. μ chains

Answer: (A)

Positive selection in the thymus is all about ensuring T cells can recognize antigens only when they are presented by the body's own MHC molecules. Developing T cells (thymocytes) traffic through the thymic cortex where they encounter self-peptide fragments bound to MHC molecules on thymic epithelial cells. If a thymocyte’s T-cell receptor binds these self-MHC–peptide complexes with the right affinity, it receives survival signals and matures further. This process establishes MHC restriction, meaning the T cells will later recognize antigens only in the context of the body's own MHC.

Explaining why the other ideas don’t fit helps: recognizing self-antigens in general wouldn’t guarantee that the T cell will respond in the right MHC context, which is essential to proper T-cell function. Stress proteins aren’t the driving factor of this developmental checkpoint, and μ chains are components of antibodies in B cells, not involved in thymic T-cell selection.