Memory B Cells and Plasma Cells are Generated in Germinal Centers

After B cells are selected in the germinal center for those bearing high-affinity mIg for antigen displayed on follicular dendritic cells, some B cells differentiate into plasma cells and others become memory B cells (see Figure 11-17). While germinal centers are important sites of plasma-cell generation, these Ig-secreting cells are fomed in other sites as well. Plasma cells generally lack detectable membrane-bound im-munoglobulin and instead synthesize high levels of secreted antibody (at rates as high as 1000 molecules of Ig per cell per second). Differentiation of mature B cells into plasma cells requires a change in RNA processing so that the secreted form of the heavy chain rather than the membrane form is synthesized. In addition, the rate of transcription of heavy-and light-chain genes is significantly greater in plasma cells than in less-differentiated B cells. Several authors have suggested that the increased transcription by plasma cells might be explained by the synthesis of higher levels of transcription factors that bind to immunoglobulin enhancers. Some mechanism also must coordinate the increase in transcription of heavy-chain and light-chain genes, even though these genes are on different chromosomes.

As indicated above, B cells that survive selection in the light zone of germinal centers also differentiate into memory

Plasma cells

Proliferating B cells (centrocytes)

Plasma cells

Proliferating B cells (centrocytes)

Cytokines Cell Proliferation

Activated B cell (centroblast)

Proliferation cytokines: IL-2, IL-4, IL-5

Differentiation cytokines: IL-2, IL-4, IL-5, IFN-y, TGF-ß

Activated B cell (centroblast)

Proliferation cytokines: IL-2, IL-4, IL-5

Differentiation cytokines: IL-2, IL-4, IL-5, IFN-y, TGF-ß

IgG2a or IgG3

IgA or IgG2b

IgE or IgG1

FIGURE 11-19

The interactions of numerous cytokines with B cells generate signals required for proliferation and class switching during the differentiation of B cells into plasma cells. Binding of the proliferation cytokines, which are released by activated TH cells, provides the progression signal needed for proliferation of activated B cells. Similar or identical effects may be mediated by cytokines beyond the ones shown. Class switching in the response to thymus-dependent antigens also requires the CD40/CD40L interaction, which is not shown here.

cells. Some properties of naive and memory B cells are summarized in Table 11-6. Except for membrane-bound immunoglobulins, few membrane molecules have been identified that distinguish naive B cells from memory B cells. Naive B cells express only IgM and IgD; as a consequence of class switching, however, memory B cells express additional isotypes, including IgG, IgA, and IgE.

Coping with Asthma

Coping with Asthma

If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.

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