He is a white blood cell, specifically a T Lymphocyte (Memory T Cell) that works under the Killer T Division. He is tasked with storing information about any foreign entities and cells that have invaded the body before. (Source: Cells at Work Wiki)

The average human body contains about 60 trillion cells, and each of them has work to do! But when you get injured, viruses or bacteria invade, or when an allergic reaction flares up, everyone from the silent but deadly white blood cells to the brainy neurons has to work together to get through the crisis! 
 
(Source: Kodansha Comics USA)

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From Akane Shimizu, winner of Shounen Sirius' Rookie Award, comes an informative tale about anthropomorthized cells!
 
Strep throat! Hay fever! Influenza! The world is a dangerous place for a red blood cell just trying to get her deliveries finished. Fortunately, she’s not alone … She’s got a whole human body’s worth of cells ready to help out! The mysterious white blood cell, the buff and brash killer T cell, the nerdy neuron, even the cute little platelets—everyone’s got to come together if they want to keep you healthy!
 
(Source: Kodansha Comics & MangaUpdates)
 
Note: Contains the spin-off one-shot "Hataraku Saibou: Kansen-hen".

Memory T Saibou