A lymphocyte that is derived from stem cells in the bone marrow but does not mature in the thymus (compare t cell); in birds it matures in the bursa of the cloaca (hence B cell). Each B cell has a unique set of some 100 000 receptor molecules on its surface, designed to recognize a specific antigen. These B-cell receptors bind soluble antigen, such as pathogen-derived proteins or toxins, and take it into the cell. Here the internalized antigen is processed and peptide fragments derived from it are bound with MHC class II proteins, which migrate to the cell surface. These antigen-MHC class II complexes are recognized by CD4-bearing helper T cells that respond to the same antigen, causing the T cell to adhere to the B cell and stimulate it by various means, including the secretion of lymphokines, notably interleukin- 4. The B cell is prompted to undergo repeated division to form a clone of cells (i.e. clonal expansion). These mature into plasma cells, capable of producing large amounts of specific antibody (see immunoglobulin), which circulates in the blood and lymph and binds to the corresponding antigen, whether this is free in body fluids or located on the surface of a pathogen. In so doing the antibodies neutralize the pathogen, preventing it from entering body cells, or mark the pathogen for destruction by macrophages, a process called opsonization. After a few days of antibody production the plasma cells die. However, some cells from the clone remain in the form of memory cells (see immunological memory), which initiate a more rapid immune response on subsequent exposure to the same antigen. See also clonal selection theory.