B cell activation

When activated by either T-independent antigens or by interactions with T helper cells, small resting B cells are converted to large lymphoblasts and then either to plasma cells, specialized for antibody production, or long-lasting small memory cells. Within germinal centers, B cells are converted to large replicating centroblasts and then to nonreplicating centrocytes. Additional intermediate stages in the activation process are defined by cell surface protein expression and localization within the germinal center. Striking features of the germinal center activation process are the generation of frequent mutations in the Ig V regions and the switch from IgM to IgG, IgA or IgE production. Mutation greatly increases the diversity of antigen-binding sites. If mutation eliminates antigen binding, the cell has a short life, dying by apoptosis. The few cells in which mutation yields an Ig product with high affinity for the antigen are selected for survival. Antigen-selected cells may differentiate terminally into plasma cells, which then secrete a large amount of the antibody, or into small, long-lived, memory B cells that are distributed in blood and lymphoid tissues. These cells no longer express surface IgD.

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