B-Cell Development, Maturation, and Activation

B cells originate from hematopoietic stem cells in the bone marrow, which is the site of B-cell development and maturation (bursa of Fabricius in avian species). In the bone marrow, large numbers of B cells are produced daily, but only a small number eventually develop to maturity. These mature B cells leave the bone marrow (or bursa of Fabricius in birds) and enter the circulation system, settling down mainly in the spleen and lymph nodes. During B-cell development, those B cells that bind to self-antigens are induced to apoptosis, so that under normal conditions, mature B cells recognize foreign antigens and do not react with self-antigens, i.e., self-tolerance (self-tolerance). However, if self-reactive B cells are present in the body and are activated to produce autoantibodies, they would attack self-tissues and cause autoimmune disease. This chapter focuses on the basics of B-cell development, maturation, and activation.