After they mature, B-cells are present in your blood and certain parts of your body Plasma cells produce large proteins called immunoglobulins, or antibodies. B cells both originate from and mature in the bone marrow, which is the soft fatty tissue inside bones. What is the major function of B cells? B cells produce. Because a principal function of B lymphocytes is antibody production, it is Mature B cells that are activated by foreign antigen and enter. Some lymphocytes migrate to the thymus, where they mature into T cells; others remain Some of the cloned B cells differentiate into plasma cells that produce.