 This is the most common protective immunoglobulin in the adult body.After antigenic stimulation, B cells differentiate into plasma cells that secrete large quantities of soluble IgG.This is the final stage of B lymphopoiesis but it is the clincher because the plasma cells must either issue antibody close to a source of infection, or disseminate it in the blood to fight an infection at a distance or in an inaccessible part of the body.