B cells

B lymphocytes (B cells) are made and mature in the bone marrow. Each B cell has many identical antibody molecules in its cell surface membrane, acting as receptors. All the receptors on one B cell have the same antigen-binding site, complementary to one specific antigen.When a B cell meets an antigen with a complementary shape, the antigen binds to its receptors. The B cell takes in the antigen by endocytosis, digests it and displays fragments of it on MHC proteins on its surface, so the B cell itself becomes an antigen-presenting cell.Usually a B cell needs a second signal before it can divide. A T helper cell that has already been activated by the same antigen binds to the antigen presented on the B cell and releases cytokines (interleukins). These stimulate the B cell to divide rapidly by mitosis, producing a clone of identical cells.The cells of the clone differentiate into two types:
• plasma cells, which secrete large amounts of antibody with the same binding site as the original receptor. They have a lot of rough endoplasmic reticulum and a large Golgi apparatus to make and secrete this protein, and live for only a few days.
• B memory cells, which stay in the blood and lymph for years and respond rapidly if the same antigen returns.B cells carry out the humoral response, which works against pathogens and toxins in body fluids such as blood plasma and tissue fluid. They cannot reach pathogens hidden inside body cells; that is the role of T cells.