Chapter 21
The Immune System: Innate and Adaptive Body Defenses
775
21
Lymphocyte Development, Maturation,
and Activation
Te development, maturation, and activation of B and ± cells
share the five general steps shown in
Figure 21.8
.
Origin (Figure 21.8
1
)
Like all blood cells, lymphocytes orig-
inate in red bone marrow from hematopoietic stem cells.
Maturation (Figure 21.8
2
)
Lymphocytes are “educated” (go
through a rigorous selection process) as they mature. Te aim of
this education is twofold:
Immunocompetence.
Each lymphocyte must become able
(competent) to recognize its one specific antigen by binding
to it. Tis ability is called
immunocompetence
. When B or ±
cells become immunocompetent, they display a unique type
Te adaptive immune system involves three crucial types of
cells: two distinct populations of lymphocytes, plus
antigen-
presenting cells
(
APCs
).
B lymphocytes (B cells)
oversee humoral immunity.
T lymphocytes (T cells)
are non-antibody-producing lym-
phocytes that constitute the cellular arm of adaptive immunity.
APCs do not respond to specific antigens as lymphocytes do.
Instead, they play essential auxiliary roles.
Lymphocytes
Despite their differences, B and ± lymphocytes share a common
pattern of development and common steps in their life cycles.
Let’s take a look.
1
2
3
4
5
Red bone marrow
Red bone marrow
Thymus
Lymph node
Lymphocyte
precursors
Antigen
Secondary lymphoid organs
(lymph nodes, spleen, etc.)
Primary lymphoid organs
(red bone marrow and thymus)
Adaptive defenses
Humoral immunity
Cellular immunity
Origin
• Both B and T lymphocyte precursors originate in
red bone marrow
.
Maturation
• Lymphocyte precursors destined to become T cells
migrate (in blood) to the thymus and mature there.
• B cells mature in the bone marrow
.
• During maturation lymphocytes develop
immunocompetence and self-tolerance.
Seeding secondary lymphoid organs and circulation
• Immunocompetent but still naive lymphocytes leave
the thymus and bone marrow
.
• They “seed” the secondary lymphoid organs and circulate
through blood and lymph.
Antigen encounter and activation
• When a lymphocyte’s antigen receptors bind its
antigen, that lymphocyte can be activated.
Proliferation and differentiation
• Activated lymphocytes proliferate (multiply) and
then differentiate into effector cells and memory
cells.
• Memory cells and effector T cells circulate
continuously in the blood and lymph and
throughout the secondary lymphoid organs.
Figure 21.8
Lymphocyte development, maturation, and activation.
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