B cells are essential parts of the humoral immune system. They are known as B lymphocytes (See: Lymphocyte), a type of white blood cell produced by the immune system. B cells are of many types including B-1 cell, B-2 cell, regulatory B cell, follicular B cell, memory B cell, plasmablast, marginal zone B cell, and lymphoplasmacytoid cell. B cells mature in the bone marrow and have B cell receptors on their cell membranes. These receptors allow the B cells to bind with the antigens (See: Antigen) so that an antibody response can be initiated. The development of B cells happens in several stages with various gene expression patterns. In the bone marrow, the B cells are still immature. Their development is complete only after they reach the spleen. Their entry to the spleen is marked by two transition phases, which are called T1 and T2. B cells are activated in the secondary lymphoid organs like the lymph nodes present in the jaws, neck, armpits, and the inguinal regions along with the spleen. Specialized B cells work along with specialized T cells to defend cells from a particular of germ and these B cells and T cells remember the invader so that the body is prepared for the next attack. This memory aspect of the B cell is due to a property called memory B cell activation. This occurs when the B cell binds with the target antigen and is shared by the parent B cell. This allows the B cell to identify future intrusions and fight them successfully.