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.n birds, B cells mature in the bursa of Fabricius (the “B” from B cells comes from the name of this organ). B cells, unlike the other 2 classes of lymphocytes, T cells and natural killer cells, express B cell receptors (BCRs) on their cell membrane.