Page 1436 - Small Animal Internal Medicine, 6th Edition

1408 PART XIII Hematology

VetBooks.ir Trabeculae Afferent lymphatic
vessel

Paracortical area
Lymphoid follicles (T cells)
(B cells)
Subcapsular
space

Afferent Capsule
lymphatic
vessel
Cortical nodule

Medullary sinus

Efferent lymphatic vessel
FIG 88.1
Microscopic anatomy of a typical lymph node in a carnivore. (From Couto CG: Diseases
of the lymph nodes and spleen. In Ettinger SJ, editor: Textbook of veterinary internal
medicine—diseases of the dog and cat, ed 3, Philadelphia, 1989, WB Saunders.)

node. Regional lymphadenopathy is the enlargement of a infiltrate, lymphadenitides are classified as suppurative (neu-
chain of lymph nodes draining a specific anatomic area. Gen- trophils predominate), granulomatous (macrophages pre-
eralized lymphadenopathy is a multicentric lymph node dominate), pyogranulomatous (macrophages and neutrophils
enlargement affecting more than one anatomic area. Lymph- predominate), or eosinophilic (eosinophils predominate). A
adenopathies can also be classified as superficial or deep (or focal area of suppurative inflammation with marked lique-
visceral) according to their anatomic location. faction (i.e., pus) is referred to as a lymph node abscess. The
Lymph nodes enlarge as a consequence of the prolifera- agents that commonly cause the different types of lym-
tion of normal cells that normally reside in the node, or phadenitis are listed in Table 88.1.
infiltration with normal or abnormal cells. Rarely lymph Infiltrative lymphadenopathies usually result from the
nodes enlarge as a result of vascular changes (e.g., hyper- displacement of normal lymph node structures by neoplas-
emia, congestion, neovascularization, edema). tic cells and, infrequently, from extramedullary hematopoi-
When normal cells proliferate within a lymph node in esis. Neoplasms affecting the lymph nodes can be primary
response to antigenic stimuli (e.g., vaccination, infection), hematopoietic tumors or secondary (metastatic) neoplasms.
the term reactive lymphadenopathy (or lymph node hyper- Lymph node infiltration by hematopoietic malignancies (i.e.,
plasia) is used. Lymphoid and MP-AP cells proliferate in lymphoma) constitutes one of the most common causes of
response to immunologic and infectious stimuli, although generalized lymphadenopathy in dogs (see Table 88.1).
occasionally a clinician evaluates a dog or cat in which a
cause for the reactive lymphadenopathy cannot be identified. Clinical Features
Because these lymphoid structures are usually presented From a clinical standpoint, familiarization with the location
with many antigens simultaneously, the cell proliferation that and palpation characteristics of normal lymph nodes, which
occurs in reactive lymphadenopathies is polyclonal; that is, should always be evaluated during a routine physical exami-
a wide variety of morphologic types of lymphoid and MP-AP nation, is important. The following lymph nodes are palpable
cell types are present in a cytologic or histopathologic in normal dogs and cats: mandibular, prescapular (or super-
specimen. ficial cervical), axillary (in approximately half of animals),
When polymorphonuclear leukocytes or macrophages superficial inguinal, and popliteal (Fig. 88.2). Lymph nodes
predominate in the cellular infiltrate, the term lymphadenitis that are palpable only when markedly enlarged include the
is used. This is usually, but not always, a result of infectious facial, retropharyngeal, mesenteric, and iliac (sublumbar)
processes. Depending on the predominant cell type in the lymph nodes.