Human Physiology, 14th edition (2016)

The Immune System 499

some viral infections, by secreting antibodies into the blood and

lymph. Because blood and lymph are body fluids (humors), the

B lymphocytes are said to provide humoral immunity, although

the term antibody-mediated immunity is also used. T lympho-

cytes attack host cells that have become infected with viruses

or fungi, transplanted human cells, and cancerous cells. The

T lymphocytes do not secrete antibodies; they must come in

close proximity to the victim cell, or have actual physical con-

tact with the cell, in order to destroy it. T lymphocytes are there-

fore said to provide cell-mediated immunity ( table 15.4 ).

Thymus

The thymus extends from below the thyroid in the neck into the

thoracic cavity. This organ grows during childhood but gradu-

ally regresses after puberty. Lymphocytes from the fetal liver

and spleen, and from the bone marrow postnatally, seed the thy-

mus and become transformed into T cells. These lymphocytes,

in turn, enter the blood and seed lymph nodes and other organs

where they divide to produce new T cells when stimulated by

antigens.

Small T lymphocytes that have not yet been stimulated by

antigens have very long life spans—months or perhaps years.

Still, new T cells must be continuously produced to provide effi-

cient cell-mediated immunity. This is particularly important fol-

lowing cancer chemotherapy and during HIV infection (in AIDS),

when the population of T lymphocytes has been depleted. Under

these conditions, the thymus can replenish the T lymphocyte

population through late childhood. Repopulation of T lympho-

cytes occurs more slowly in adulthood, and appears to be accom-

plished mostly by production of T lymphocytes in the secondary

lymphoid organs rather than in the thymus. This is because the

thymus of adults becomes more of a fatty organ, although pro-

duction of lymphocytes in the thymus has been demonstrated to

occur to some extent in people over the age of 70.

Secondary Lymphoid Organs

The secondary lymphoid organs include about 450 lymph nodes,

as well as the spleen, tonsils, and areas called Peyer’s patches

under the mucosa of the intestine (chapter 13; see fig. 13.38).

These organs are strategically located across epithelial membranes

in areas where antigens could gain entry to the blood or lymph.

The spleen filters blood, whereas the other secondary lymphoid

organs filter lymph received from lymphatic vessels.

Secondary lymphoid organs capture and concentrate

pathogens, present them to macrophages and other cells, and

serve as sites where circulating lymphocytes can come into

contact with the foreign antigens. Lymphocytes move from

the primary lymphoid organs (bone marrow and thymus) to

the blood, and exit the blood through specialized postcapillary

venules in lymph nodes to travel in the lymph from lymphoid

organ to lymphoid organ. This ceaseless travel increases the

likelihood that a given lymphocyte, specific for a particular

antigen, will be able to encounter that antigen. This process is

aided, particularly in the case of T lymphocytes, by other cells

that are known as antigen-presenting cells (see fig.  15.13 ).

Antigen-presenting cells, chiefly dendritic cells, also migrate

Lymphocytes and Lymphoid Organs

Leukocytes, erythrocytes, and blood platelets are all ultimately
derived from (“stem from”) unspecialized cells in the bone
marrow. These stem cells produce the specialized blood cells,
and they replace themselves by cell division so that the stem
cell population is not exhausted. Lymphocytes produced in this
manner seed the thymus, spleen, and lymph nodes, producing
self-replacing lymphocyte colonies in these organs.
The lymphocytes that seed the thymus become
T lymphocytes, or T cells (the letter T stands for thymus-
dependent ). These cells have surface characteristics and
an immunological function that differ from those of other
lymphocytes. The thymus, in turn, seeds other organs; about
65% to 85% of the lymphocytes in blood and most of the
lymphocytes in the germinal centers of the lymph nodes and
spleen are T lymphocytes. T lymphocytes, therefore, either
come from or had an ancestor that came from the thymus.
Most of the lymphocytes that are not T lymphocytes are
called B lymphocytes, or B cells. The letter B derives from
immunological research performed in chickens. Chickens have
an organ called the bursa of Fabricius that processes B lympho-
cytes. Since mammals do not have a bursa, the B is often trans-
lated as the bursa equivalent for humans and other mammals.
It is currently believed that the B lymphocytes in mammals are
processed in the bone marrow, which conveniently also begins
with the letter B. Because the bone marrow produces B lympho-
cytes and the thymus produces T lymphocytes, the bone marrow
and thymus are considered the primary lymphoid organs.
Both B and T lymphocytes function in specific immunity.
The B lymphocytes combat bacterial infections, as well as

Figure 15.4 An immunoassay using the
agglutination technique. Antibodies against a particular
antigen are adsorbed to latex particles. When these are mixed
with a solution that contains the appropriate antigen, the
formation of the antigen-antibody complexes produces clumping
(agglutination) that can be seen with the unaided eye.

Latex

particles

+ Antigen X

X

X

X

X

X

Anti-X

antibodies

Antibodies attached to latex particles

Agglutination (clumping) of latex particles