Chapter 13
Innate and Adaptive Immunity
303
organs. Lymphocytes are produced and undergo maturation
in the central lymphoid organs (bone marrow and thymus)
and are subsequently stored in the peripheral lymphoid
structures (lymph nodes, spleen, mucosa-associated lym-
phoid tissues in the respiratory, gastrointestinal, and
reproductive systems) where they function to concentrate
antigen, aid in the processing of antigen, and promote cel-
lular interactions necessary for development of adaptive
immune responses.
Adaptive immunity can be acquired actively or pas-
sively. Active immunity develops through immunization
or by having a disease, while passive immunity develops
when the host receiving antibodies or immune cells from
another source. An acquired immune response can improve
with repeated exposure to an injected antigen or a natural
infection.
DEVELOPMENTAL ASPECTS
OF THE IMMUNE SYSTEM
After completing this section of the chapter, you should
be able to meet the following objectives:
••
Explain the transfer of passive immunity from
mother to fetus and from mother to infant during
breast-feeding.
••
Characterize the development of active immunity in
the infant and small child.
••
Describe the changes in the immune response that
occur during the normal aging process.
Development of the immune system begins early in fetal life
at approximately 5 to 6 weeks gestation when the fetal liver
actively begins hematopoiesis.At about the same time (6 weeks
gestation), the thymus arises from the third branchial arch with
the cortex arising from its ectodermal layer and the medulla
from the endoderm.
37,38
Over the next 2 to 3 weeks, lymphoid
cells initially migrate from the yolk sac and fetal liver and
then from the bone marrow to colonize the fetal thymus.
37,38
Development of the secondary lymphoid organs (
i.e.,
spleen,
lymph nodes, and mucosa-associated lymphoid tissues) begins
soon after. The secondary lymphoid organs are rather small but
well developed at birth and mature rapidly after exposure to
microbes during the postnatal period. The thymus is the largest
lymphoid tissue in the neonate relative to body size and nor-
mally reaches its mature weight by 1 year of age.
Transfer of Immunity from
Mother to Infant
The neonate’s immune system is functionally immature at
birth so protection against infection and toxic substances
occurs through transfer of maternal IgG antibodies. Maternal
IgG antibodies readily cross the placenta during fetal
IN SUMMARY
Adaptive immunity is comprised of two distinct but inter-
related processes: cell-mediated and humoral immunity.
Together they respond to foreign antigens, amplify and sus-
tain immunological responses, distinguish self from non-
self, and confer “memory” so that a heightened response
can be initiated on subsequent exposure to an organism.
Antigens are usually substances foreign to the host that can
stimulate an immune response. Antigens possess specific
antigenic binding sites for the cells of the immune system
known as epitopes. Epitopes allow the adaptive immune
system to distinguish foreign antigens from normal cellular
substances whose destruction would be detrimental to the
organism.
The principal cells of the adaptive immune system are
the B and T lymphocytes, APCs, and effector cells that are
responsible for the elimination of antigens. B lymphocytes
differentiate into plasma cells that produce antibodies and
provide for the elimination of microbes in the extracellular
fluid (humoral immunity) as well as memory cells, which
are responsible for the rapid immune response with repeat
exposure. T lymphocytes differentiate into regulatory
(helper T and regulatory T cells) and effector (cytotoxic
T cells) cells. APCs consist of macrophages and DCs that
process and present antigen peptides to CD4
+
helper T cells.
During cellular maturation, T lymphocytes express spe-
cific CD molecules on the cellular surfaces that distinguish
between the different cell types and that help determine the
cells’ functionality. Regulatory CD4
+
helper T cells help
to modulate the immune response and are essential for the
differentiation of B cells into antibody-producing plasma
cells and the differentiation of T lymphocytes into effector
CD8
+
cytotoxic T cells. The CD8
+
cytotoxic cells eliminate
intracellular microbes, such as viruses and other pathogens.
Cells of both the innate and adaptive immune responses pro-
duce cytokines that influence adaptive immune responses.
These cytokines function as communication molecules for
the B and T lymphocytes, stimulate cellular proliferation
and differentiation, and ensure the appropriate development
of cytotoxic effector and memory cells.
Essential to the proper functioning of the adaptive
immune response are the cell surface MHC molecules that
allow the immune system cells to distinguish self from
nonself. Class I MHC complexes that are present are body
cells other than those of the immune system, interact with
cytotoxic T cells, and present degraded viral protein frag-
ments from infected cells for destruction.
Class II MHC
(MHC-II) complexes are found on immune cells includ-
ing phagocytic APCs, immune cells that engulf foreign
particles such as macrophages and DCs. Class II MHC
complexes also aid in cell-to-cell communication between
different cells of the immune system.
The cells of the adaptive immune system are present
in large numbers in the central and peripheral lymphoid
