then migrate into the tissues where they establish
themselves as fixed defenders called macrophages.
Granulocytes circulate in the blood and in the
lymph and also take up residence in the lymph
structures and the general body tissues. The three
subtypes of granulocytes—basophils, eosinophils,
and neutrophils—have specified roles in the
body’s inflammatory response and are responsible
for hypersensitivity reactions and allergies. The
bone marrow primarily manufactures leukocytes,
with assistance from the lymph tissues and spleen
when necessary to meet the body’s INFECTIONcon-
trol needs.
Flow with the body: the lymph In contrast to
the force of the blood’s circulation, the lymph
channels through the body at a gentle flow of
about 100 milliliters per hour. Gravity and the
body’s movements (MUSCLEcontractions) massage
lymph through the lymph vessels that roughly
parallel the arteries and veins. The lymph vessels
are thin-walled, originating with cul-de-sac struc-
tures arising in the epithelial spaces, the lymphatic
capillaries, that join increasingly larger channels
that carry lymph into the central body and ulti-
mately into the circulating blood.
Slightly more watery than blood (92 percent),
lymph carries a suspension of primarily lympho-
cytes and monocytes as well as dissolved proteins
and other substances. Clear and only slightly more
viscous than water, lymph drains from the spaces
between cells into the lymphatic capillaries,
microscopic channels comprised of a single thick-
ness of cells overlapped like backward shingles.
This construction encourages fluid to seep under
the cells and into the lymph capillaries. The lymph
capillaries collect the droplets of lymph, pooling
them into microscopic trickles that eventually
merge with larger lymphatic vessels—the CISTERNA
CHYLI, THORACIC DUCT, andRIGHT LYMPHATIC DUCT—
that carry the lymph toward the subclavian veins
where it rejoins the bloodstream.
The lymph carries leukocytes, proteins, anti-
bodies, and other materials directly to the cells.
While erythrocytes in the blood can carry oxygen
molecules into the CAPILLARY BEDS, the capillaries
eventually become too narrow even for the flexi-
ble erythrocytes to make further passage. So the
erythrocytes off-load their oxygen molecules into
the lymph, which floats them through the capil-
lary walls and into the interstitial spaces (the
space between the cells of the tissues). Lymph
flows through the interstitial spaces, bathing the
cells, which then withdraw the nutrients, includ-
ing oxygen and glucose, that they require. Cells
also discharge their metabolic wastes into the
lymph.
Critical passengers in the lymph are the leuko-
cytes, predominantly neutrophils and lympho-
cytes. These protective cells vigilantly patrol the
interstitial spaces on the alert for invading
pathogens. When they detect pathogenic invaders
leukocytes secrete chemicals, called CYTOKINES, into
the lymph that initiate or activate specific immune
responses. Some of these responses recruit addi-
tional lymphocytes and granulocytes (notably
neutrophils) into circulation both in the lymph
and in the blood. As agents of immune response,
granulocytes and lymphocytes have the ability to
migrate between the blood and the tissues, bol-
stering the body’s defenses as needed.
The lymph also transports pathogens, such as
viruses and bacteria, to the lymph nodes where
masses of lymphocytes, macrophages (tissue-
bound monocytes), and granulocytes wait to dis-
pose of them. The lymph nodes often swell when
they are busy fighting infections (LYMPHADENOPA-
THY) and may themselves become infected (LY M-
PHADENITIS). The lymph also offers a route of
transport for cancer cells that leave the original
tumor site. The lymph network can unfortunately
carry cancer cells that enter its flow to any loca-
tion within the body, facilitating METASTASIS
(spread of the cancer).
Teaching the concept of self: the thymus The
thymus, a loose structure of lymph tissue behind
the sternum (breastbone) in the center of the
chest, functions somewhat as a prep school for
immature T-cells. In the thymus, T-cells learn to
distinguish between “self” and “nonself,” a funda-
mental concept of immune function that prevents
the IMMUNE SYSTEM from attacking cells that
belong to the body. The thymus releases those that
learn the lesson, and they migrate into the lymph
tissues where they can reside for many years. T-
cells that fail to recognize self and nonself do not
gain release, and they die without leaving the thy-
mus. In AUTOIMMUNE DISORDERSsuch as MYASTHENIA
GRAVISand SYSTEMIC LUPUS ERYTHEMATOSUS(SLE), it120 The Blood and Lymph