Hataraku Saibou!!

Synopsis

U-1146, Normal Cell, and NK Cell go to the large intestine, which is full of toxic gas due to harmful bacteria. AE3083 joins the platelets on a tour of the large intestine. The main Killer T Cell has been promoted to a Memory T Cell and joins them in their search for Cancer Cell. U-1146 tells Normal Cell to go alone and find where the last lactic acid bacterium belongs. Harmful bacteria invade the large intestine, forcing Normal Cell to run while AE3083 helps the platelets evacuate. Cancer Cell ambushes U-1146, NK Cell, and Memory T Cell. They slay him, but it turns out to be a copy. The real one appears and beats them up, saying they will see which of them deserves to survive.

(Source: Wikipedia)