When we are exposed to a pathogen, the immune system's B cells swarm to our lymph nodes, spleens, and tonsils. There, those cells mutate in germinal centers—microscopic boot camps that rush the B cells through volleys of mutations to produce the most potent antibodies for neutralizing the infectious agent. As long as a germinal center is up and running, B cells are free to mature and perfect their approach to fighting disease. But when a germinal center shuts down, usually after a few weeks, the training process grinds to a halt. Whatever antibodies happen to have formed by then are, for better or worse, the immune system's final product.
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