184
UNIT 3
Organ Systems
Figure 10-3.
A representation of B-lymphocyte maturation.
H&E,
3
83
B lymphocytes
(
B cells
) originate and mature in the bone marrow. Because
naive
(
virgin
)
B lymphocytes
differentiate from
precursor cells
(
pro–B lymphocytes
), they become randomly programmed to recognize a
speci± c antigenic determinant
. Dur-
ing the B-cell maturation process, they are subjected to
negative selection
, through which those B cells that happen to recognize
self-antigens are induced to undergo
apoptosis
. Naive B lymphocytes are
immunocompetent cells
with speciF
c
antibodies
(
Igs
)
inserted into their plasma membrane as receptors. Each B lymphocyte has the ability to recognize and respond to a particular
antigen
. After newly matured B lymphocytes leave the bone marrow, they use the vasculature and their own motility to recirculate
through the peripheral lymphoid organs (lymph nodes, spleen, MALT, etc.). This continual wandering increases the likelihood that
a lymphocyte will encounter its antigen if the antigen has gained entry into the body. Naive B cells die in a few days or weeks if
they do not meet their antigen, but those that encounter their speciF
c antigen under favorable conditions will become activated. B
cells that are activated by an encounter with antigens undergo cell division and differentiation. Some descendants of an activated
B cell become
memory B cells
; others differentiate into
effector B cells
, that is,
plasma cells
, which are able to produce and secrete
antibodies
. Antibodies secreted by plasma cells become widely distributed throughout the body so that foreign antigens are unlikely
to evade binding by antibodies and the defense mechanisms that are triggered by antibody binding. Memory B cells have a much
longer life than naive B cells; they enter the blood circulation in an inactive state and may live and recirculate for decades. If there is a
subsequent encounter with the same antigen, memory B cells rapidly divide and differentiate into plasma cells that secrete antibodies
in great quantity, thereby producing a much quicker and more powerful
secondary immune response
.
Pro–B lymphocytes
Pro–B lymphocytes
in bone marrow
in bone marrow
Pro–B lymphocytes
in bone marrow
Circulation
Activated
B cells
Plasma cell in the
lymphoid organs and
connective tissue
Antibodies secreted into blood,
lymph, or connective tissue
Memory B cells in the
circulation
and
lymph organs
M
B lymphocytes in
lymph nodes, spleen,
and other lymphoid organs
Binding antigens to antibody;
inactive B lymphocytes
become activated B cells
IgE
IgM
IgD
IgG
First
encounter
with an
antigen
Second
encounter
with same
antigen
IgA
SYNOPSIS 10-1
Characteristics of Types of Immunoglobulins
There are F ve types of Igs classiF
ed by their heavy chains:
IgG
: This is the most abundant type of Ig in blood serum and the only one that is able to cross the placenta. It is a major
Ig during the secondary immune response and has high antigen-binding speciF
city.
IgA
: This is the major type of Ig in external secretions (milk, saliva, tears, sweat, and mucus) of epithelial cells, including
gland epithelial cells. Its main function is to protect mucosal (epithelial) surfaces. It includes subclasses IgA1 and IgA2.
IgM
: This is the principal Ig in the primary immune response; it is most effective in activating a complement but with lower
antigen-binding speciF city. It activates macrophages and serves as an antigen receptor on the B cell surfaces.
IgE
: This is found only in small amounts in blood serum; it binds to ±c receptors of the mast cells and basophils and plays
an important role in allergic reactions (see mast cell, ±ig. 4-4B).
IgD
: This has a low concentration in blood serum; it serves along with IgM as an antigen receptor on the membranes of
mature B cells.
B Lymphocytes
previous page 199 Dongmei Cui -  Atlas of Histology with Functional and Clinical Correlations 2011 read online next page 201 Dongmei Cui -  Atlas of Histology with Functional and Clinical Correlations 2011 read online Home Toggle text on/off