B Cell Differentiation: From Bone Marrow to Plasma and Memory Cells

B cells are lymphocytes that mature in the bone marrow. In peripheral immune organs, antigens transmit antigen recognition signals to B cells by binding to the B cell receptor (BCR). Additionally, B cells act as specialized antigen-presenting cells (APCs) to activate T-helper (Th) cells, and they are activated by obtaining a second signal through T-B cell interaction. Activated B cells have two differentiation pathways. The first pathway is the entry into the medullary cords and differentiation into primary foci containing plasma cells, producing primarily IgM antibodies. Although most of these B cells undergo apoptosis within two weeks, they provide early defense against infections to the body. The second pathway involves migration to primary lymphoid follicles with follicular helper T (Tfh) cells, followed by continued proliferation and the formation of germinal centers. Within the microenvironment of the germinal center, B cells undergo clonal expansion and interact with Tfh cells and follicular dendritic cells (FDCs). This process involves somatic hypermutation, affinity maturation, and immunoglobulin class switching, leading to the differentiation of plasma cells and memory B cells.