Chapter 21
The Immune System: Innate and Adaptive Body Defenses
779
21
Immunological Memory
Te cellular proliferation and differentiation we have just de-
scribed constitute the
primary immune response
, which
occurs on first exposure to a particular antigen. Te primary re-
sponse typically has a lag period of 3 to 6 days aFer the antigen
encounter. Tis lag period mirrors the time required for the few
B cells specific for that antigen to proliferate (about 12 genera-
tions) and for their offspring to differentiate into plasma cells.
AFer the mobilization period, plasma antibody levels rise, reach
peak levels in about 10 days, and then decline
(Figure 21.12)
.
If (and when) someone is reexposed to the same antigen,
whether it’s the second or twenty-second time, a
secondary
immune response
occurs. Secondary immune responses are
faster, more prolonged, and more effective, because the immune
system has already been primed to the antigen, and sensitized
memory cells are already “on alert.” Tese memory cells provide
what is commonly called
immunological memory
.
Within hours aFer recognizing the “old enemy” antigen, a
new army of plasma cells is being generated. Within 2 to 3 days
the antibody concentration in the plasma, called the
antibody
titer
, rises steeply to reach much higher levels than in the primary
response. Secondary response antibodies not only bind with
greater affinity (more tightly), but their blood levels remain high
for weeks to months. (When the appropriate chemical signals are
present, plasma cells can keep functioning for much longer than
the 4 to 5 days seen in primary responses.) Memory cells persist
Now that you understand the common steps in lymphocyte
maturation and activation, let’s examine how this basic pattern
applies to B lymphocytes. When a B cell encounters its antigen,
that antigen provokes the
humoral immune response
, in which
antibodies specific for that antigen are made.
Activation and Differentiation of B Cells
An immunocompetent but naive B lymphocyte is
activated
when matching antigens bind to its surface receptors and cross-
link adjacent receptors together. Antigen binding is quickly
followed by receptor-mediated endocytosis of the cross-linked
antigen-receptor complexes. As we described previously, this is
called
clonal selection
and is followed by proliferation and dif-
ferentiation into effector cells
(Figure 21.11)
. (As we will see
shortly, interactions with ± cells are usually required to help B
cells achieve full activation.)
Most cells of the clone differentiate into
plasma cells
, the
antibody-secreting
effector cells
of the humoral response.
Plasma cells develop the elaborate internal machinery (largely
rough endoplasmic reticulum) needed to secrete antibodies at
the unbelievable rate of about 2000 molecules per second. Each
plasma cell functions at this breakneck pace for 4 to 5 days and
then dies. Te secreted antibodies, each with the same antigen-
binding properties as the receptor molecules on the surface of
the parent B cell, circulate in the blood or lymph. Tere they
bind to free antigens and mark them for destruction by other
innate or adaptive mechanisms.
Clone cells that do not become plasma cells become long-
lived
memory cells
. Tey can mount an almost immediate hu-
moral response if they encounter the same antigen again in the
future (²igure 21.11, bottom).
Table 21.3
Overview of B and T Lymphocytes
 
B LYMPHOCYTES
T LYMPHOCYTES
Type of immune
response
Humoral
Cellular
Antibody
secretion
Yes
No
Primary targets
Extracellular patho-
gens (e.g., bacteria,
fungi, parasites, some
viruses in extracellular
fluid)
Intracellular
pathogens (e.g., virus-
infected cells) and
cancer cells
Site of origin
Red bone marrow
Red bone marrow
Site of
maturation
Red bone marrow
Thymus
Effector cells
Plasma cells
Cytotoxic T (T
C
) cells
Helper T (T
H
) cells
Regulatory T (T
Reg
)
cells
Memory cell
formation
Yes
Yes
Antibody titer (antibody concentration)
in plasma (arbitrary units)
10
4
10
3
10
2
10
1
10
0
0
7
14
21
28
35
42
49
56
Time (days)
Anti-
bodies
to A
First exposure
to antigen A
Second exposure to antigen A;
first exposure to antigen B
Anti-
bodies
to B
Primary immune
response
to antigen
A occurs after a delay.
Secondary immune response
to
antigen A is faster and larger;
primary
immune response
to antigen B is
similar to that for antigen A.
Figure 21.12
Primary and secondary humoral responses.
The
primary response to antigen A generates memory cells that give rise
to the enhanced secondary response to antigen A. The response to
antigen B is independent of the response to antigen A.
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