Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

1310 Chapter 24: The Innate and Adaptive Immune Systems
antibody-secreting
effector Bβ cells
secreted antibodies
lymphoid
progenitor cell
different
resting
B cells
antigen
Bβ
Bα Bβ Bγ
Bβ
Bβ
Bβ
PROLIFERATION AND B CELL
DIVERSIFICATION IN BONE MARROW
ANTIGEN BINDING TO SPECIFIC B CELL (Bβ)
IN PERIPHERAL LYMPHOID ORGAN
PROLIFERATION (CLONAL EXPANSION)
AND DIFFERENTIATION OF Bβ CELLS
activates the lymphocyte; this causes the lymphocyte to proliferate, thereby producing
many more cells with the same receptor—a process called clonal expansion.
The encounter with antigen also causes some of the cells to differentiate into
effector cells. An antigen therefore selectively stimulates those cells that express
complementary antigen-specific receptors and are thus already committed to
MBoC6 m25.08/24.16
respond to it (Figure 24–15). This arrangement, called clonal selection, provides
an explanation for immunological memory, whereby we develop lifelong immunity
to many common infectious diseases after our initial exposure to the pathogen—either
through natural infection or vaccination.
It is easy to demonstrate such immunological memory in experimental animals.
If an animal is immunized once with antigen A, an immune response (antibody,
T‐cell-mediated, or both) can be detected after several days; the response
rises rapidly and exponentially, and then, more gradually, declines. This is the
characteristic course of a primary immune response, occurring on an animal’s
first exposure to an antigen. If, after some weeks, months, or even years have
elapsed, the animal is immunized again with antigen A, it will usually produce
a secondary immune response that differs from the primary response: the lag
period is shorter, because there are now many more preexisting B or T cells (or
both) with specificity for antigen A, and the response is greater and more efficient.
These differences indicate that the animal has “remembered” its first exposure
to antigen A. If the animal is given a different antigen (for example, antigen B)
instead of a second immunization with antigen A, the response is typical of a primary,
and not a secondary, immune response. The secondary response therefore
reflects antigen-specific immunological memory for antigen A (Figure 24–16).
Immunological memory depends on both lymphocyte proliferation and
differentiation. In an adult animal, the peripheral lymphoid organs contain a
mixture of lymphocytes in at least three stages of maturation: naïve cells, effector
cells, and memory cells. When naïve cells encounter their specific foreign antigen
for the first time, the antigen stimulates some of them to proliferate and differentiate
into effector cells, which then carry out an immune response (effector B cells
secrete antibody, whereas effector T cells either kill infected cells or influence
the response of other immune cells—by secreting cytokines, for example). Some
of the antigen-stimulated naïve cells multiply and differentiate into memory
cells, which are more easily and more quickly induced to become effector cells
by a later encounter with the same antigen: like naïve cells, when memory cells
encounter their antigen, they give rise to either effector cells or more memory
cells (Figure 24–17).
Bβ
Figure 24–15 Clonal selection. An antigen
activates only those lymphocytes that are
already committed to respond to it. The
committed cell expresses cell-surface
receptors that specifically recognize the
antigen. The human adaptive immune
system consists of many millions of
different T and B lymphocyte clones,
with cells within a clone expressing the
same unique antigen receptor. Before
its first encounter with antigen, a clone
would usually contain only one or a small
number of cells. A particular antigen may
activate hundreds of different clones, each
expressing a different antigen receptor that
binds either a different part of the antigen
or the same part with a different binding
affinity. Although only B cells are shown
here, T cells are selected in a similar way.
Note that the antigen receptors on the
B cells labeled β in this diagram have the
same antigen-binding site as the antibodies
secreted by the effector Bβ cells. As we
discuss later, B cells require co-stimulatory
signals from T cells to become activated by
antigen to proliferate and differentiate into
antibody-secreting cells (not shown).