Immune Reactions Occur in
the Lymph Nodes, Spleen,
and Lymphatic Nodules
As you saw in Figure 12.2, the structures of the lymphatic
system are divided into primary and secondary tissues.
P rim a ry ly m p h a tic tissues,
which include the thymus and red
bone marrow, are responsible for making the cells that
perform the immune functions of
the lymphatic system.
These
major
immune
cells
are the B and T
lymphocytes
and
phagocytic cells. The phagocytic
cells destroy cellular debris and
foreign cells that get into the sys-
tem. Lymphocytes produce
anti-
bodies
that help protect against
foreign antigens (molecules that stimulate immune re-
sponses). You will learn more about these cells in the
next section.
The
secondary lym p h a tic tissues
are locations throughout
the body where various immune cells reside and where
immune reactions occur. They include the lymph nodes,
spleen, and lymphatic nodules. Let’s take a look at the
lymph nodes first (Figure 12.4).
Just as sewage goes through a filtration/treatment
plant, the lymph is filtered and immunologically processed
in the lymph nodes to remove foreign substances. Remov-
al of debris and disease-causing organisms (also known as
pathogens
) from the lymph greatly reduces the incidence
of disease. This ability to protect against disease is known
as
immunity
. Because of these functions, the lymphatic
system is sometimes referred to as the
im m u n e system .
lymphocytes
(LIM-
fo-sTtz) A type of white
blood cell that helps
carry out cell-mediated
and antibody-mediat-
ed immune responses;
found in blood and
lymphatic tissues.
The stru c tu re and function o f th e lym ph n o d e •
Figure 12.4
Lymph nodes are multilayered structures that filter lymph.
Types of cells in
a
lymph node
Lymph node has the following
layers:
• Capsule— Dense
connective tissue
• Cortex (outer, inner)
• Medulla
Different layers contain
various combinations of
lymphocytes and other cells
>
Efferent lymphatic
vessels
\
-
Valves keep lymph
flowing in one direction.
Reticular fibers
between cells trap
foreign substances.
Lymphocytes participate
in immune responses,
either by killing cells
directly or secreting
antibodies.
Antigen-presenting cells
including dendritic cells and
macrophages ingest foreign
cells and cellular debris
using phagocytosis and
present the foreign proteins
(antigens) to lymphocytes
for making antibodies.
Afferent
lymphatic vessels
348 CHAPTER 12
The Lymphatic System and Immunity
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