Download Answers to WHAT DID YOU LEARN QUESTIONS

CHAPTER 24
Answers to “What Did You Learn?”
1.
An immune response is a systematic defense against antigens (abnormal
substances to the body) by lymphatic cells. Some lymphatic cells produce soluble
proteins, antibodies that bind to and destroy the antigen while other lymphatic
cells attack and destroy the antigens directly.
2.
Lymph is the combination of interstitial fluid, solutes, and sometimes foreign
material that has entered the lymphatic capillaries.
3.
The smallest lymph vessels are lymphatic capillaries. Lymphatic capillaries are
closed-ended tubes that are interspersed among most blood capillary networks
except those within the red bone marrow and central nervous system. A
lymphatic capillary is similar to a blood capillary in that its wall is an
endothelium, however, it is larger in diameter than blood capillaries, lacks a
basement membrane, and has overlapping endothelial cells. Lymphatic
capillaries have one-way flaps that allow lymph to enter but not escape, and these
capillaries eventually drain into lymphatic vessels.
4.
The right lymphatic duct receives lymph from the lymphatic trunks that drain the
right side of the head and neck, right upper limb, and right side of the thorax.
5.
The types of lymphatic cells are macrophages, some epithelial cells (called nurse
cells), dendritic cells, and lymphocytes.
6.
T-lymphocyte types include helper T-lymphocytes that are needed to begin an
effective defense against antigens, initiate and then oversee the immune response;
cytotoxic T-lymphocytes, which come into direct contact with infected or foreign
24-1
cells and kill them; memory T-lymphocytes arise from T-lymphocytes that have
encountered a foreign antigen, patrol the body and if they encounter the same
antigen again, they mount an even faster immune response; and regulatory Tlymphocytes appear to turn off the immune response once it has been activated to
help regulate its performance. B-lymphocyte types include activated lymphocytes
that produce antibodies, and memory B-lymphocytes that mount an even faster
and more powerful immune response at the next encounter with the antigen.
7.
Lymphopoiesis is the process of lymphocyte development and maturation. All
lymphocytes originate from a hemopoietic stem cell found in the red bone
marrow. T-lymphocytes mature in the thymus while B-lymphocytes mature in the
red bone marrow.
8.
MALT (mucosa–associated lymphatic tissue) is composed of large collections of
lymphatic nodules located in the mucosa and found in the GI tract, respiratory
tract, and genitourinary tract.
9.
The thymus functions as a site for T-lymphocyte maturation and differentiation.
T-lymphocytes within the thymus do not participate in the immune response and
are protected from antigens by a well-formed blood–thymus barrier around the
blood vessels in the cortex.
10.
Lymph nodes are small, round or ovoid structures located along the pathways of
lymph vessels. Each lymph node is surrounded by a tough connective tissue
capsule with internal extensions called trabeculae. The tissue deep to the lymph
node capsule is subdivided into an outer cortex and an inner medulla. Lymph
nodes function to filter lymph and mount an immune response.
24-2
11.
The white pulp is associated with the arterial supply of the spleen and consists of
circular clusters of lymphatic tissue (T-lymphocytes, B-lymphocytes, and
macrophages). The red pulp is associated with the venous supply of the spleen. It
consists of splenic cords and splenic sinusoids. The white pulp mounts an
immune response when needed, while the red pulp serves as a reservoir for blood
as well as phagocytize aged erythrocytes and platelets.
12.
The reduced number of lymphocytes in an elderly person means that the body’s
ability to acquire immunity and resist infection decreases. In addition, the
lymphocytes that remain appear less able to target malignant cells, which may be
one reason why the elderly are more prone to developing cancers.
13.
The spleen begins to form during the fifth week of embryonic development.
Answers to “Content Review”
1.
The functions of the lymphatic system include reabsorbing and transporting
excess interstitial fluid back to the bloodstream, transporting dietary lipids and
lipid-soluble vitamins from the intestine to the bloodstream, storing and assisting
in lymphocyte development, and generating an immune response.
2.
Lymph is interstitial fluid that is transported in the lymph vessels. Once lymph
enters the lymphatic capillaries, it is transported to and from lymph nodes via
lymphatic vessels, lymphatic trunks, and finally lymphatic ducts before being
emptied into the venous circulation. Each lymphatic duct drains at the junction of
the internal jugular and subclavian veins.
3.
The thoracic duct drains most regions of the body, including the left side of the
24-3
head and neck, left upper limb, left thorax, and all body regions inferior to the
diaphragm, including the right lower limb and right side of the abdomen.
4.
Macrophages are monocytes that have migrated from the bloodstream into
lymphatic structures. They are responsible for phagocytosis of foreign substances
and may present antigens to the other lymphatic cells. Special epithelial cells
(also called nurse cells) are found in the thymus and secrete thymic hormones.
Dendritic cells are found in lymphatic nodules; they internalize antigens from the
lymph and present them to other lymphatic cells. These cells are the main
antigen-presenting cell of the immune system. Lymphocytes are the most
abundant cells in the lymphatic system. There are different types of lymphocytes:
T-lymphocytes, B-lymphocytes, and natural killer (NK) cells. All migrate
through the lymphatic system and search for antigens. (1) T-lymphocytes: helper
T-lymphocytes initiate and manage the immune response; cytotoxic Tlymphocytes kill by secreting substances into foreign or abnormal cells; memory
T-lymphocytes mount an even faster immune response at the next encounter with
the antigen; and regulatory T-lymphocytes turn off the immune response. (2) Blymphocyte types include plasma cells that produce antibodies, and memory Blymphocytes that mount an even faster immune response at the next encounter
with the antigen. Natural killer (NK) cells are large, granular lymphocytes that
kill infected or cancerous cells.
5.
Helper T-lymphocytes recognize an antigen and then stimulate the production of
cytotoxic T-lymphocytes and B-lymphocytes. The cytotoxic T-lymphocytes will
mount a cellular attack against foreign cells while B-lymphocytes will form
24-4
plasma cells that produce antibodies against the antigens. Both types of
lymphocytes produce memory cells that will recognize the antigen and help
mount a faster and more powerful immune response if the antigen reappears.
6.
Lymphatic nodules are ovoid clusters of lymphatic cells with some extracellular
connective tissue matrix that are not surrounded by a connective tissue capsule.
The pale center of a lymphatic nodule is called the germinal center, which
contains proliferating B-lymphocytes and some macrophages. T-lymphocytes are
located outside the germinal center. Individually, a lymphatic nodule is small.
However, sometimes lymphatic nodules will group together, forming larger
structures such as mucosa-associated lymphatic tissue (MALT) or tonsils.
7.
In infants and young children, the thymus is quite large and extends into the
superior mediastinum. The thymus continues to grow until puberty, when the
organ reaches a maximum weight of 30–50 grams. During this time, the thymus
is the site for maturing and differentiating T-lymphocytes. By adulthood, the Tlymphocyte maturation process is complete and differentiated T-lymphocytes are
produced only by mitosis of existing T-lymphocytes. Because the adult thymus is
no longer an active T-lymphocyte maturation site, the thymic tissue regresses and
the functional thymus is replaced by adipose connective tissue. In adults, the
thymus atrophies.
8.
Each lymph node is surrounded by a tough connective tissue capsule and has
internal extensions called trabeculae projecting into the node, subdividing it into
compartments. Deep to the lymph node capsule there is an outer cortex and an
inner medulla. The cortex consists of lymphatic nodules and cortical sinuses
24-5
(lymphatic sinuses found in the cortex). The medulla contains strands of
lymphatic tissue (consisting primarily of B-lymphocytes and macrophages) called
medullary cords and medullary sinuses (lymphatic sinuses found in the medulla).
The function of the lymph node is to filter the lymph, and mount an immune
response if antigens are found in the lymph.
9.
The arterial supply to the spleen is associated with the white pulp and consists of
circular clusters of T-lymphocytes, B-lymphocytes, and macrophages. The white
pulp detects antigens in the blood and mounts an immune response if antigens are
found. The red pulp is associated with the venous drainage of the spleen. It is
formed by splenic cords and splenic sinusoids. The red pulp is a reservoir of
erythrocytes and the macrophages here remove aged or damaged erythrocytes.
10.
The lymph vessels and lymph nodes first start to form during week six of
embryonic development, when the primary lymph sacs form. Paired lymphatic
vessels connect the lymph sacs by the ninth week of development. Eventually, a
single thoracic duct forms, and the right lymphatic duct develops as well, and new
lymph vessels continue to develop during and after the embryonic period. During
the fetal period, the lymph sacs develop into rounded lymph nodes.
24-6