B cells are a type of white blood cell that play a crucial role in the immune system. They are responsible for producing antibodies, which are proteins that help identify and neutralize harmful invaders like bacteria and viruses. When B cells encounter a pathogen, they can quickly multiply and differentiate into plasma cells, which are specialized for antibody production. Once B cells have produced antibodies, they can also form memory B cells. These memory cells remain in the body long after an infection has cleared, allowing the immune system to respond more rapidly if the same pathogen invades again. This is why vaccines are effective; they help the body create these memory cells without causing illness.