Page 1435 - Small Animal Internal Medicine, 6th Edition

CHAPTER 88
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Lymphadenopathy and

Splenomegaly

APPLIED ANATOMY AND HISTOLOGY a lymph node reacting to a bacterial infection has primarily
B-cell hyperplasia characterized by increased numbers of
The lymph nodes and spleen constitute the main source of secondary follicles. This histologic-functional compartmen-
immunologic and mononuclear-phagocytic (MP) cells in the talization should be kept in mind when interpreting cyto-
body. Because these lymphoid structures are in a constant logic or histopathologic lymph node specimens.
dynamic state, they continuously reshape and change in size Recently, Suami et al. described lymphatic territories
in response to antigenic stimuli. In general, the response of (lymphosomes), areas that drain specific lymph nodes and
the cells within a lymph node to different stimuli is similar are useful when identifying sentinel lymph nodes during
to that occurring in the spleen. However, the spleen responds oncologic surgery (Suami et al., 2013).
primarily to bloodborne antigens (mainly nonopsonized
organisms), whereas the lymph nodes respond to antigens
arriving through the afferent lymphatics (i.e., local tissue FUNCTION
response). The response of the lymph nodes and spleen to
different stimuli is briefly reviewed in this chapter. The two main functions of the lymph nodes are to filter
Lymph nodes are reniform, encapsulated, well-developed particulate material and participate in immunologic proc-
structures responsible for filtering lymph and participating esses. Particulate material is filtered as lymph flows through
in immunologic reactions. Fig. 88.1 depicts the basic micro- the areas rich in MP cells while it moves from the afferent to
scopic anatomy of a lymph node in a carnivore. It is com- the efferent lymphatics. During this transit, particulate mate-
posed of a capsule, subcapsular spaces, cortex, paracortex, rial is taken up and processed by the MP or antigen-
and medulla. Each of these areas has specific functions. The processing (AP) cells and presented to the lymphoid cells to
capsule surrounds and supports all other structures within elicit a humoral or cellular immune response.
the node (stroma). The subcapsular spaces (or sinuses) The spleen has multiple functions, including extramedul-
contain mainly MP cells responsible for “filtering” particles lary hematopoiesis, filtration and phagocytosis, remodeling
arriving through the afferent lymphatics and presenting the of red blood cells (RBCs), removal of intraerythrocytic
antigens to the lymphoid cells. The cortex contains mainly inclusions, storage of RBCs and platelets, metabolizing of
B-cell areas in the germinal centers; when properly stimu- iron, and immunologic functions. The canine spleen also
lated, the primary follicles turn into secondary follicles, con- stores reticulocytes and releases them into circulation in
taining primarily early lymphoid cells in the center. The response to catecholamine release (Horvath et al., 2014).
paracortex is composed primarily of T cells and is therefore Because of its nonsinusal nature, the feline spleen is less
involved in cell-mediated immunity. The medulla contains efficient at removing intracellular inclusions than its canine
the medullary cords, in which the committed B cells persist counterpart.
and may expand to solid areas of plasma cells in response to
antigenic stimulation. Between the medullary cords, the
medullary sinuses form an endothelial sieve containing LYMPHADENOPATHY
varying numbers of MP cells, which screen the efferent
lymph. The lymph flows from the medulla to the efferent Etiology and Pathogenesis
lymphatics in the hilus. In this chapter, lymphadenopathy is defined as lymph node
An understanding of the different histologic and func- enlargement. According to the distribution, the following
tional characteristics of these anatomic areas aids in under- terms are used to characterize lymphadenopathy. Solitary
standing the pathogenesis of lymphadenopathy. For example, lymphadenopathy refers to the enlargement of a single lymph

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