Page 324 - Anatomy and Physiology of Farm Animals, 8th Edition

Body Defenses and the Immune System / 309

cells routinely develop in the body but are vena cava or some of its tributaries. The
smallest lymphatics begin blindly between
soon recognized and destroyed by the killer
VetBooks.ir cells that recognize the abnormal antigens. tissue cells as lymphatic capillaries,
which collect the tissue fluid not absorbed
This process of recognition and destruction
is termed immunological surveillance. by the venous system. When the tissue
Clinically evident cancers develop when fluid enters the lymphatic vessels, it is
this surveillance is inadequate. known as lymph, which consists of fluid
originally derived from the blood, a variety
of blood cells, and sometimes microbes.
Lymphatic System Lymphatic vessels carry lymph back to the
great veins of the heart.
The lymphatic system includes the
lymphoid tissues (e.g., lymph nodes and
nodules and the spleen) and the lymphatic Lymph Nodes
vessels distributed throughout the body
(Fig. 16‐2). It drains excess interstitial Lymph nodes are discrete knots of lym-
tissue fluid (called lymph within the lym- phoid tissue scattered along the course of
phatic system) and is a framework for the lymph vessels. Lymph nodes filter the
circulation, production, and maturation of lymph and act as one of the first defenses
immune cells. The tissue fluid–draining against infection by harboring lympho-
function of the lymphatic system augments cytes, plasma cells, and macrophages.
the venous circulation and therefore assists Each lymph node is surrounded by a
in the control of interstitial fluid pressures connective tissue capsule that sends
and helps to maintain normal oncotic pres- numerous connective tissue trabeculae
sure within the extravascular space. The into the substance of the node (Fig. 16‐3).
lymphatic system is also an important The node is roughly divided into cortex,
component of immunologic defense of the paracortex, and medulla, with large num-
body, as movement of lymph brings micro- bers of lymphocytes and macrophages in
organisms and other foreign substances all three. Lymphocytes in the cortex are
into contact with immune cells. arranged in nodules (Fig. 16‐3). Dark‐
Lymphoid tissue consists of accumula- staining groups are primary nodules, and
tions of lymphocytes trapped in the spaces light‐staining groups are secondary nod-
between fibers of reticular connective tis- ules. Secondary nodules are areas of rapid
sue. The lymphoid tissue may be scattered B cell proliferation, and for that reason
diffusely in some regions (as is commonly their cores are called germinal centers.
found in mucous membranes), may appear The paracortex, deep to the cortex, is pop-
as nodules (as in the intestinal submucosa), ulated primarily with T lymphocytes and
or may be encapsulated to form specific dendritic cells.
organs, including lymph nodes, the spleen, Lymphocytes in the medullary portion
thymus, and tonsils. The lymphatic vessels of the lymph node are arranged in medul-
and tissues are arranged so that tissue fluid lary cords rather than nodules. These tend
is exposed to aggregates of immune cells, to be primarily accumulations of plasma
which scrutinize the fluid for foreign cells cells.
and substances, thereby assisting in the Immediately deep to the capsule of the
control of infection. node is a space, the subcapsular sinus,
which communicates with other sinuses of
Lymphatic Vessels the cortex and medulla. Lymph delivered
by afferent lymph vessels enters the sub-
The lymphatic vessels constitute a one‐way capsular sinus and is slowly filtered through
pathway that parallels the venous system the cortex and medulla, to emerge finally at
and eventually empties into the cranial the hilus of the node, where blood vessels