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chromatin pattern. Single to multiple nucleoli are often visible. Plasma cells, macrophages, neutrophils and mast cells are occasionally seen in very low numbers in normal nodes.
Reactive Lymphoid Hyperplasia (RLH), or Reactive Lymph Node.
In a reactive node, small, well-differentiated lymphocytes are still the predominant population, but increased numbers of intermediate lymphocytes and Increased numbers of lymphoblasts is usually present, particularly in the cat. However, the lymphoblast population
typically will not exceed 10 to 20% of the total lymphoid population of a reactive node. The most striking feature in reactive nodes from dogs is the presence of Plasma cells. Plasma cells are medium-sized round to oval cells with a single eccentrically placed round nucleus. The nucleus of a mature plasma cell is the same size and color as a small lymphocyte but the cytoplasm is much more abundant. The cytoplasm is deeply basophilic and generally have a visible Golgi apparatus appearing as
a clear area located between the nucleus and greatest volume of cytoplasm.
Inflammation (lymphadenitis).
In lymphadenitis, the predominant non-lymphoid inflammatory cell population categorizes the type of inflammation present. Suppurative inflammation is characterized by the presence of increased numbers of neutrophils beyond what may be expected from any blood contamination present. Here, hreater than 5% of nucleated cells are neutrophils. This is usually the result of a bacterial infection either in the node (abscessed lymph node) or in an area being drained. Eosinophilic inflammation is characterized by an inflammatory reaction that contains an eosinophilic infiltration, usually accompanied by a mild increased numbers
of neutrophils +/- low numbers of macrophages. An eosinophilic lymphadenitis is most commonly caused
by an allergic dermatitis, and is typically seen in the inguinal or popliteal lymph nodes. Other common causes of eosinophilic lymphadenitis include other non-dermatologic allergic/hypersensitivity reactions, eosinophilic granuloma complex, parasitic diseases, eosinophilic gastroenteritis, hypereosinophilic syndrome, and mast cell tumors. In rare cases, lymphoma cells may secrete chemotactic factors that result in an eosinophilic infiltration. Pyogranulomatous inflammation contains
a significant macrophage component, with or without the presence of neutrophils. This type of inflammation typically results from fungal infections (blastomycosis, coccidioidomycosis, cryptococcosis, or sporotrichosis) protozoal infections (cytauxzoonosis, toxoplasmosis,
or leishmaniasis) mycobacterial infections, Nocardia/Actinomyces, Bartonella in dogs. A mild pyogranulomatous inflammation may also be observed in lymph nodes that drain areas of chronic inflammation
An Urban Experience
or neoplasia. (Figure on right is pyogranulomatous lymphadenitis with Blastomycosis organisms)
Lymphoid neoplasia (lymphoma).
Lymphoma is suspected whenever 30% of the cells population from a lymph node aspirate is lymphoblasts, though, typically, the lymphoid population will likely be between 50% to 90%. When >50% lymphoblast cells are present, a cytological diagnosis of lymphoma can be reliably made. Lymphomas may be classified by their tissue of origin (e.g. renal, thymic, intestinal etc.), with multicentric lymphoma being the most common type observed in dogs. However, knowing the “cytologic type” of lymphoma present may give some indication
of the grade of malignancy, the potential for response
to chemotherapy, and the potential, or explanation for paraneoplastic syndromes such as hypercalcemia. The most accurate means of typing lymphoma is by using lymphocyte markers that will determine the subset of lymphocytes involved in the neoplastic process (e.g. B-cells, T-cells such as CD4 or CD8, or Natural Killer cells).
Canine Lymphoma
In canine lymphoma, the predominant cell type
is the immature lymphoblast. Only rarely will the
small, well-differentiated lymphocytes become neoplastic. Lymphoblasts are large cells with nuclei that vary in size from 2 to 5 times the size of erythrocytes with a deeply basophilic cytoplasm that is more abundant than that of small or intermediate lymphocytes. The chromatin pattern is more diffuse and paler staining than in the well-differentiated lymphocyte. A variable number of distinct or indistinct nucleoli are frequently visible. (Figure right, canine lymphoma).
Note: The previous administration of glucocorticoids can drastically alter the lymphocytes population within a lymph node. Lymphoblasts are very sensitive to the cytotoxic effects of glucocorticoids much more so than mature lymphocytes. This may iatrogenically decrease the differential lymphoblast count below 30% to 50% of the population therefore making a lymphoma diagosis difficult.
Feline Lymphoma
The same criteria for diagnosing lymphoma are used
in dogs and cats. When a lymph node aspirate or
mass is aspirated and found to contain a population
of lymphocytes of which 50% or more are blast cells, lymphoma can reliably be diagnosed. However, two complicating factors make the diagnosis of lymphoma in the cat more difficult than in the dog: 1. Lymphomas in the cat are more frequently composed of a population of well-differentiated lymphocytes, which is rarely observed in dogs, and 2. as mentioned previously, a condition known as “Distinctive Peripheral Lymph Node Hyperplasia” (DPLH) that clinically, cytologically and

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