Maintenance of the Body
Other Lymphoid Organs
Unlike lymph nodes, the spleen, thymus, tonsils, and Peyer’s
patches do not ﬁlter lymph. Most lymphoid organs contain both
macrophages and lymphocytes.
Te spleen provides a site for lymphocyte proliferation and
immune function, and destroys aged or defective red blood
cells and bloodborne pathogens. It also stores and releases the
breakdown products of hemoglobin as necessary, stores platelets,
and acts as a hematopoietic site in the fetus.
Te thymus is most functional during youth. Te thymus
provides the environment in which ± lymphocytes mature and
Mucosa-Associated Lymphoid Tissue (MALT)
Peyer’s patches of the intestinal wall, lymphoid follicles of the
appendix, tonsils of the pharynx and oral cavity, and follicles
in the genitourinary and respiratory tract mucosae are known
as MAL± (mucosa-associated lymphoid tissue). Tey prevent
pathogens in these passages from penetrating the mucous
Immune System; Topic: Anatomy Review, pp. 13–14, 16–20.
Developmental Aspects of the Lymphatic System
and Lymphoid Organs and Tissues
Lymphatics develop as outpocketings of developing veins. Te
thymus develops from endoderm; the other lymphoid organs
derive from mesenchymal cells of mesoderm.
Te thymus is the ﬁrst lymphoid organ to develop.
Lymphoid organs are populated by lymphocytes, which arise
from hematopoietic tissue.
Lymphoid Cells and Tissues
Te cells in lymphoid tissues include lymphocytes (cells called ±
cells or B cells), plasma cells (antibody-producing oﬀspring of B
cells), macrophages and dendritic cells (cells that capture antigens
and initiate an immune response), and reticular cells that form
the lymphoid tissue stroma.
Lymphoid tissue is reticular connective tissue. It houses
macrophages and a continuously changing population of
lymphocytes. It is an important element of the immune system.
Lymphoid tissue may be diﬀuse or packaged into dense follicles.
Follicles o²en display germinal centers (areas where B cells are
Immune System; Topic: Anatomy Review, pp. 4–5
Lymph nodes, the principal lymphoid organs, are clustered along
lymphatic vessels. Lymph nodes ﬁlter lymph and help activate the
Structure of a Lymph Node
Each lymph node has a ﬁbrous capsule, a cortex, and a medulla.
Te cortex contains mostly lymphocytes, which act in immune
responses; the medulla contains macrophages, which engulf
and destroy viruses, bacteria, and other foreign debris, as well as
lymphocytes and plasma cells.
Circulation in the Lymph Nodes
Lymph enters the lymph nodes via aﬀerent lymphatic vessels
and exits via eﬀerent vessels. Tere are fewer eﬀerent vessels;
therefore, lymph ﬂow stagnates within the lymph node, allowing
time for its cleansing.
Immune System; Topic: Anatomy Review, pp. 10–12.
(Some questions have more than one correct answer. Select the best
answer or answers from the choices given.)
serve as sites for immune surveillance,
transport leaked plasma proteins and ﬂuids
to the cardiovascular system,
are represented by vessels that
resemble arteries, capillaries, and veins.
Te saclike initial portion of the thoracic duct is the
right lymphatic duct,
Entry of lymph into the lymphatic capillaries is promoted by which
of the following?
one-way minivalves formed by overlapping
the respiratory pump,
the skeletal muscle
greater ﬂuid pressure in the interstitial space.
Te structural framework of lymphoid organs is
Lymph nodes are densely clustered in all of the following body
Te germinal centers in lymph nodes are largely sites of
proliferating B lymphocytes,
all of these.
Te red pulp areas of the spleen are sites of
macrophages, and red blood cells,
connective tissue septa.
Te lymphoid organ that functions primarily during youth and
then begins to atrophy is the
Collections of lymphoid tissue (MAL±) that guard mucosal
surfaces include all of the following except
Short Answer Essay Questions
Compare and contrast blood, interstitial ﬂuid, and lymph.
Compare the structure and functions of a lymph node to those of
(a) Which anatomical characteristic ensures that the ﬂow of
lymph through a lymph node is slow? (b) Why is this desirable?
Tere are no lymphatic arteries. Why isn’t this a problem?