Which statement best explains how molecular mimicry and bystander activation can contribute to autoimmunity after infection or vaccination?

• A. Molecular mimicry may trigger autoimmunity when immune responses cross-react with self; bystander activation can activate autoreactive lymphocytes during infection or vaccination.
• B. Bystander activation prevents activation of autoreactive lymphocytes.
• C. Molecular mimicry always protects against autoimmunity.
• D. Autoimmunity is not influenced by infections or vaccination.

Answer: (A)

Two processes underlie how infections or vaccines can tip the immune system toward autoimmunity: molecular mimicry and bystander activation. Molecular mimicry occurs when a foreign antigen shares structural features with a self antigen, so the immune response against the pathogen cross-reacts with self tissues and can initiate autoimmune attack. Bystander activation happens when the inflammatory environment during infection or vaccination boosts antigen-presenting cell activity and cytokine signaling, which can awaken autoreactive lymphocytes that were previously dormant, leading to tissue damage. This combination is why the statement is correct: it captures both cross-reactivity and immune-activation contexts that can drive autoimmunity after infection or vaccination. It would be inconsistent to say bystander activation prevents autoreactive lymphocytes from activating, or that molecular mimicry always protects against autoimmunity, or that infections or vaccines don’t influence autoimmunity.