Red Blood Cell AE3803 and White Blood Cell U-1146 are back at work inside the same body of 37 trillion cells, where a delivery route can turn into a fight without warning. This second season puts the pair and the platelets against a tougher class of invader and gives more room to the cells nobody notices, from the ordinary cells doing routine work to the bacteria that live in the gut. The body's own habits cause as much trouble as any germ.