Which of the following would NOT contribute to autoimmunity?

• A. clonal deletion of self-reactive T cells
• B. molecular mimicry
• C. increased expression of class II MHC antigens
• D. polyclonal activation of B cells

Answer: (A)

Eliminating self-reactive T cells in the thymus is a key tolerance process that reduces the autoimmune T-cell pool. Because this clonal deletion removes autoreactive T cells, it prevents self-directed immune attacks rather than promoting them. In contrast, molecular mimicry can trigger an autoimmune reaction when immune responses against a pathogen cross-react with host proteins. Increased expression of class II MHC antigens boosts presentation of self-peptides to helper T cells, potentially activating autoreactive clones and promoting autoimmunity. Polyclonal activation of B cells can non-specifically expand various B-cell clones, including those that produce autoantibodies, fueling autoimmune disease.