Page 457 - Veterinary Immunology, 10th Edition

VetBooks.ir Memory B Cells

One reason that the primary immune response ends is that the
responding B cells and plasma cells are simply removed by

apoptosis. If all these cells died, however, immunological memory
could not develop. Clearly some B cells must survive as memory
cells. B cells are activated by antigen and helper T cells in the
paracortex of lymph nodes. Most of these B cells differentiate into
plasma cells and migrate to the bone marrow, spleen, and other

organs, but some memory precursors remain in the cortex,
proliferate, and form germinal centers. (Asymmetric division, as
described above, likely accounts for these two different fates.) These

cells persist under the influence of programming and rescue
signals. Thus, memory cells are first screened for their ability to
bind antigen. This induces CD154 on nearby T cells, which in turn
promotes expression of bcl-2. Bcl-2 protects them against apoptosis
and allows them to differentiate into memory cells.

Memory cells form a reserve of long-lived antigen-sensitive cells
to be called on following subsequent exposure to an antigen. There
are several classes of memory B cells distinguishable by their

immunoglobulin class, their location, and their passage through
germinal centers. For example, one population consists of small,
long-lived resting cells with IgG BCR. These cells, unlike plasma
cells, look like generic lymphocytes. Their survival does not depend
on antigen contact. On exposure to antigen, they proliferate and

differentiate into plasma cells without undergoing further
mutation. It has been calculated that in a secondary immune
response, the clonal expansion of memory B cells results in 8- to 10-

fold more plasma cells than does a primary immune response.
A second memory B cell population consists of large, dividing
cells with IgM BCR. These cells persist in germinal centers, where
their continued survival depends on exposure to antigen on
follicular dendritic cells. There are two distinct populations of

plasma cells: a short-lived population that lives for 1 to 2 weeks and
produces large amounts of antibodies shortly after antigen
exposure, and a long-lived population that can survive for months

or years. (In humans these plasma cells have a half-life of 8 to 15

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