Blood Cells - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 155

3.12.1 B-Lymphocyte Subpopulations

B-lymphocyte types include: (1) short-lived plasma B lymphocytes, or plasmocytes
(plasma cells), that have been exposed to antigen and produce and secrete large
amounts of antibodies; (2) long-lived memory B lymphocytes that derive from
activated B cells and are specific to the antigen encountered during the primary
immune response; (3) B1 lymphocytes that express IgM in greater quantities than
IgG [ 285 ], hence having polyspecific, low-affinity receptors, and reside in lymph
nodes and spleen in small amounts, but lodge predominantly in the peritoneal
and pleural cavities; (4) B2 lymphocytes that synthesize IgG1 precursors more
(

7 times) than IgM precursors [ 285 ]; (5) marginal-zone (MZ) B lymphocytes that
are mature, naive, non-circulating B cells; and (6) naive follicular B lymphocytes
that circulate until they encounter antigen or die.
Immature B cells produce plasmalemmal immunoglobulin-M. They leave the
bone marrow and differentiate into B1 and conventional B2 cells with distinct
locations and activation modes. B-cell clones that reach the mature long-lived pool
include the B1 subset, CD21 high compartment (mostly marginal zone), whereas oth-
ers recirculate primarily among B-cell follicles. This segregation depends on B-cell
receptors and B-lymphocyte coreceptor CD19 [ 284 ]. B-lymphocyte subpopulations
B1a, B1b, B1c (

>

α M β 2 -integrin − and CD5 + ), and B2 have different roles during

infections.

3.12.2 B-Lymphocyte Maturation Stages

B lymphocytes mature into naive B lymphocytes that express plasmalemmal IgM
and IgD via 2 transitional stages. During the second maturation stage, some
B lymphocytes home to and remain in the marginal zone of the spleen to form a
pool of mature, naive, non-circulating, marginal-zone B cells .
The remaining part of the B-cell population (most B cells) mature into naive
follicular B cells that circulate to the spleen follicles and lymph nodes via the
bone marrow until they encounter cognate antigen or die. Follicular B cells that are
activated by antigen, possibly in cooperation with T cells, form germinal centers
of secondary lymphoid follicles and their antibody-encoding genes are modified.
Some germinal center B cells express an immunoglobulin IgG-type BCR; others
immunoglobulin IgM-type BCR. These 2 BCR types prime different signaling.
When follicular B cells receive costimulation from activated T cells, they can
develop into plasmablasts or long-life memory B cells . Memory B cells that can
quickly react to presentation of other antigen types express antibodies that are not
secreted.
Germinal centers are thus sites where memory B cells and plasmocytes are gen-
erated for long-lasting, antibody-mediated (humoral) immunity. T-cell-dependent
immunity relies on the selection of high-affinity B cells and depends on high-rate