As the primary cell type present in lymphoid organs is lymphocytes, the first part of this handout will describe the morphologic
features of lymphocytes and then briefly describe other methods to assess lymphocytes. The second part of the handout will
describe the interpretation of various lymphoid organs.
Small lymphocytes are smaller in size than a neutrophil and have a round nuclei that takes up the majority of the cell. The
nuclei contain densely aggregated chromatin forming large chromocenters (condensed chromatin). Nucleoli are not seen. The
cytoplasm is scant (sometimes only a very thin rim is visible) and lightly basophilic in color. These are typically called
'mature lymphocytes'. However, early lymphoid progenitor cells, hematopoietic stem cells, certain stages and types of dendritic
cells, and other immature precursor cells may have a very similar morphology to 'mature, well-differentiated, small, resting
Intermediate to large lymphocytes range in size from slightly larger than small lymphocytes to the size of neutrophils. The
nuclei still takes up the majority of the cell, however more abundant cytoplasm is visible in these cells. Often, the nuclei
is placed eccentrically within the cytoplasm. The nuclear chromatin is finely clumped to granular. Typically, nucleoli are
not seen although strands of loosely clumped nuclear chromatin may be mistaken for nucleoli. The cytoplasm is lightly basophilic
in color. Occasionally these cells contain azurophilic granules suggestive of a natural killer (NK) phenotype.
Lymphoblasts are as large as a neutrophil or larger. Size alone does not indicate neoplasia. Very large lymphoblasts (2-4x
the size of neutrophils) may be seen in reactive and hyperplastic processes. Lymphoblasts contain round to oval nuclei with
fine or stippled chromatin (loosely aggregated chromatin). One or more nucleoli may be visible. The cytoplasm is moderately
to deeply basophilic. Occasionally (seen more in cats than dogs) the cytoplasm may contain punctate vacuoles.
Reactive lymphocytes are similar in morphology to small lymphocytes but are slightly larger and have more abundant, more basophilic
Plasma cells are intermediate sized cells that contain small, round, eccentrically placed nuclei with condensed chromatin.
Cytoplasm is abundant, deeply basophilic, and often contains a prominent, eccentric, perinuclear, clear zone that corresponds
to the Golgi.
Morphologic features and typical findings used to characterize atypical lymphocytes
Lymphoglandular bodies are round, homogeneous, basophilic structures comprised of cytoplasmic fragments. The presence of lymphoglandular
bodies is seen in cytologic preparations of lymphoid tissue that contains increased numbers of lymphoblasts. This can be due
to neoplasia (lymphoma) or hyperplasia.
The presence of an eccentric, perinuclear clearing zone is often suggested as a feature of B-cells and plasma cells. The clearing
zone is the Golgi and it is a prominent feature in plasma cells. However, the Golgi apparatus is an organelle found in most
cells, including T-cells and myeloid cells.
In humans, Sezary cells are described as medium to large lymphocytes with ceribriform nuclei. These neoplastic T-cells are
characteristic features of Sezary syndrome which encompasses mycosis fungoides, an epitheliotropic variant of cutaneous lymphoma.
A similar syndrome occurs in dogs but has been rarely reported in cats. In dogs, epitheliotropic T-cell lymphoma is also seen
in the gastrointestinal tract. T-cells predominate in both the cutaneous and GI variants. Interestingly, expression of protein
gene product 9.5 (PGP 9.5), a marker previously considered specific for neural and neuroendocrine tissues, was recently detected
in over 8/14 cases of canine cutaneous mycosis fungoides suggesting that there may be other biologic differences between the
human and canine variants.
In humans, the presence of flower cells or cloverleaf cells is most often associated with T-cell disease and is particularly
a feature of infection with human T-lymphotrophic virus-1 (HTLV-1). In dogs and cats, similar morphology has been seen in
both B-cell and T-cell lymphoproliferative disease as well as myeloproliferative disease.
In humans, CLL is considered a disease of B-cells. In contrast, CLL of dogs and cats is primarily a T-cell disease. In dogs,
CD8+ (cytotoxic) CLL predominate while in cats, CD4+ (T-helper) CLL is more common. However, there is variation in the disease
in both dogs and cats and B-cell, CD4+ T-cell, and CD8+ T-cell CLL have all been diagnosed in small animals.
Chronic lymphocytosis: In dogs, a chronic lymphocytosis comprised of intermediate sized lymphocytes with small azurophilic
granules has been reported in association with Ehrlichiosis.