CD45 cells are the “memory cells” of the immune system. Some of them, called RA cells, have been activated by past exposure to toxins; they quickly respond to environmental threats they have encountered before. The RO cells, in contrast, are kept in reserve for new challenges; they are turned on to deal with threats the body has not met previously. The RA-to-RO ratio is the balance between cells that recognize known toxins and cells that wait for new information to activate.
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