The Immune System: Innate and Adaptive Body Defenses
Lymphocyte Development, Maturation,
Te development, maturation, and activation of B and ± cells
share the ﬁve general steps shown in
Origin (Figure 21.8
Like all blood cells, lymphocytes orig-
inate in red bone marrow from hematopoietic stem cells.
Maturation (Figure 21.8
Lymphocytes are “educated” (go
through a rigorous selection process) as they mature. Te aim of
this education is twofold:
Each lymphocyte must become able
(competent) to recognize its one speciﬁc antigen by binding
to it. Tis ability is called
. 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
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.
Despite their diﬀerences, B and ± lymphocytes share a common
pattern of development and common steps in their life cycles.
Let’s take a look.
Red bone marrow
Red bone marrow
Secondary lymphoid organs
(lymph nodes, spleen, etc.)
Primary lymphoid organs
(red bone marrow and thymus)
• Both B and T lymphocyte precursors originate in
red bone marrow
• 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
• Memory cells and effector T cells circulate
continuously in the blood and lymph and
throughout the secondary lymphoid organs.
Lymphocyte development, maturation, and activation.