B Cell is a male lymphocyte whose primary function is to serve as an antibody-producing cell. His role is to create and deploy antibodies to fight against antigens, which include bacteria and other foreign invaders. He is first seen early in the story and is recognized by his distinctive uniform, which consists of a blue hat, jacket, and pants, similar in design to the outfits worn by white blood cells. He carries a tank filled with antibodies on his back, which is shaped like a drum bag, and wields a shooting weapon with a Y-shaped barrel, a design that changes each time he appears.

In terms of personality, B Cell is portrayed as a bright and hot-blooded young man. He is cheeky and possesses a high level of confidence in his own abilities, showing a clear preference for receiving praise while harboring a strong dislike for being ignored. He is also known to be somewhat oblivious to the atmosphere, which can lead to conflicts with other cells.

His primary motivation is to effectively eliminate pathogens using the antibodies he produces. He depends on another lymphocyte, Memory Cell, to supply him with the necessary information so he can create the specific antibody required to combat each unique antigen. This partnership is essential, as his antibody production relies on the data provided by Memory Cell.

Throughout the story, B Cell plays a key role in the immune response. One of his most notable relationships is with Memory Cell, with whom he works closely and even shares living quarters. Their dynamic is crucial, though not without its difficulties, as Memory Cell can be scatterbrained and prone to panic. For instance, during an invasion of the mumps virus, B Cell had to physically strike Memory Cell to snap him out of a delusional state and focus on the task of recalling past immunological data from a vaccine. He also has a notably poor relationship with mast cells. During a disaster caused by cedar pollen allergies, he had a fierce argument with a mast cell over which of them was responsible for the allergic reaction. On other occasions, such as when Campylobacter invaded the small intestine, several of his fellow B cells appeared on the Peyer's patches as part of a coordinated ambush.

B Cell shows some development in his abilities. While his work requires time to prepare antibodies, which can delay his response to an invasion, the results are highly effective at terminating bacteria. He is also shown to upgrade his signature antibody gun, making it lighter so it feels almost weightless. Despite his confidence, he can sometimes be a coward; after losing the specificity of his antibodies in one battle, he fled, and following a reprimand during the cedar pollen allergy incident, he attached a charm to his gun for good luck.