Colorized transmission electron micrograph of a B cell from a human donor. B cells, also known as B lymphocytes, are a type of white blood cell of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system by secreting antibodies. B cells present antigen (they are also classified as professional antigen-presenting cells (APCs) and secrete cytokines. In mammals, B cells mature in the bone marrow, which is at the core of most bones. B cells, unlike the other two classes of lymphocytes, T cells and natural killer cells, express B cell receptors (BCRs) on their cell membrane. BCRs allow the B cell to bind a specific antigen, against which it will initiate an antibody response.