According to the Centers for Disease Control and Prevention, B cells are "white blood cells that help the body defend itself against infection. These cells are produced in bone marrow and develop into plasma cells that produce antibodies. B cells are also known as B lymphocytes.
A B cell produces a range of antibodies that are displayed on its cell surface. When the cell takes up a pathogen or foreign antigens by endocytosis, these select which antibody the cell will produce and stimulate it to secrete these proteins into the blood. This process is called "B cell activation".