Germinal centers arise within 7-10 days after initial exposure to a thymus-dependent antigen. During the first stage of germinal-center formation, activated B cells undergo intense proliferation. These proliferating B cells, known as centro-blasts, appear in human germinal centers as a well-defined dark zone (Figure 11-17). Centroblasts are distinguished by their large size, expanded cytoplasm, diffuse chromatin, and absence or near absence of surface Ig. Centroblasts eventually give rise to centrocytes, which are small, nondividing B cells that now express membrane Ig. The centrocytes move from the dark zone into a region containing follicular dendritic cells called the light zone, where some centrocytes make contact with antigen displayed as antigen-antibody complexes on the surface of follicular dendritic cells. Three important B-cell differentiation events take place in germinal centers: affinity maturation, class switching, and formation of plasma cells and memory B cells. In general, affinity maturation and memory-cell formation require germinal centers. However some class switching and significant plasma-cell formation occur outside germinal centers.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.