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Pathology of the Lymphoid System
Learning Objectives:
●Define lymphadenitis and enumerate its types.
●Briefly describe the morphological appearance of reactive lymph node.
●Describe the microscopic picture of the lymph node affected by acute & chronic
inflammation.
●Classify tumors of the lymph node and spleen.
●Give simple classification of non-Hodgkin's lymphoma. Briefly describe an example of each
subgroup.
●Give an account on Hodgkin's disease with emphasis on histological classification &
staging.
●Enumerate common causes of enlargement of the spleen.
●Define hypersplenism and enumerate its causes.
Lymph Node:
I. Non-Neoplastic Conditions:
►Reactive Lymphadenitis: Any immune response against foreign antigens (infectious &
noninfectious inflammatory stimuli) is often associated with lymph node enlargement
(lymphadenopathy). The infections that cause lymphadenitis are numerous and varied
and may be acute or chronic. In most instances, the histological appearance of the nodes
is entirely nonspecific i.e. different etiologies are associated with similar microscopic
changes.
■Acute Nonspecific Lymphadenitis:
Grossly: inflamed nodes are swollen & congested i.e. gray-red.
Microscopically: there are large germinal centers containing numerous mitotic figures. When
the cause is a pyogenic organism, a neutrophilic infiltrate is seen about the follicles and
within the lymphoid sinuses. With severe infections, the centers of follicles can undergo
suppurative necrosis. The overlying skin is frequently red, and penetration of the infection to
the skin can produce draining sinuses.
■Chronic Nonspecific Lymphadenitis:
This condition can assume one of three patterns, depending on the causative agent:
1. Follicular hyperplasia
2. Paracortical hyperplasia
3. Sinus histiocytosis
●Follicular Hyperplasia: is associated with infections or inflammatory processes that
activate B cells in the B-cell areas i.e. the follicles, & thus create the follicular (or germinal
center) reaction. The cells in the reactive follicles include the activated B cells (called
follicular center cells), scattered phagocytic macrophages containing nuclear debris and
follicular dendritic cells (function in antigen display to the B cells).
Causes of follicular hyperplasia include:
*Rheumatoid arthritis. *Toxoplasmosis. *Early stages of HIV infection.
●Paracortical Hyperplasia: The paracortex is the zone situated between the cortex and the
medulla, which contains the mobile pool of T lymphocytes responsible for cell-mediated
immune responses. Paracortical hyperplasia is characterized by reactive changes within the T-
cell regions of the lymph node, which is reflected microscopically as expanded zones between
the cortical follicles. On immune activation parafollicular T-cells transform into large
proliferating immunoblasts.
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Paracortical hyperplasia is encountered in:
*Viral infections (such as EBV). *Following certain vaccinations. *Immune reactions
induced by certain drugs.
●Sinus Histiocytosis: is characterized by distention and prominence of the lymphatic
sinusoids, owing to a marked hypertrophy of lining endothelial cells and an infiltrate of
macrophages.
Sinus histiocytosis is often encountered in:
*Lymph nodes draining cancers and may represent an immune response to the tumor or
its products.
■Granulomatous lymphadenitis:
Immune reactions are usually involved in the development of granulomas. There is often
strong activation of T lymphocytes leading to macrophage activation, which can cause injury
to normal tissues. Granulomatous inflammation is a distinctive pattern of chronic
inflammation that is encountered in a limited number of infectious and some noninfectious
conditions. The causes of granulomatous lymphadenitis include:
●Infections. ●Foreign body reactions. ●Malignancy.
Among the infectious causes are: *Tuberculosis. *Atypical mycobacteriosis.
*Sarcoidosis. *Fungal infections. *Toxoplasmosis. *Syphilis & Leprosy. *Brucellosis.
●Malignancy related granulomatous lymphadenitis: occurs whether the neoplasm is:
*Primary (Hodgkin & nonHodgkin lymphomas), or
*Secondary (metastatic carcinoma).
It occurs whether the node is involved by the malignancy or not.
II. Lymphoid Neoplasms:
All lymphoid neoplasms have the potential to spread to lymph nodes and various tissues
throughout the body, especially the liver, spleen, and bone marrow. In some cases lymphomas
spill over into the peripheral blood, creating a leukemia-like picture. Conversely, leukemias of
lymphoid cells, originating in the bone marrow, can infiltrate lymph nodes and other tissues,
creating the histologic picture of lymphoma.
Two groups of lymphomas are recognized:
■Hodgkin lymphoma and (HL).
■Non-Hodgkin lymphomas (NHL).
According to the Iraqi Cancer Registry (2009); NHL ranks the 6
th
, and represents 4.83% of
the top ten cancers by primary site in Iraq.
The behavior and treatment of Hodgkin lymphoma differ from those of most NHLs,
thus making the distinction between the two of practical importance.
It has been shown that NHLs is often widely disseminated at the time of diagnosis, even
when the disease appears clinically localized, that is why only systemic therapies are
curative.
In contrast, Hodgkin lymphoma often presents at a single site and spreads in a
predictable fashion to neighboring lymph node groups. For this reason, early in the
disease, local therapy may be effective.
The WHO has established a widely accepted classification that depends on a combination of:
*Clinical. *Microscopic. *Immunophenotypic. *Genetic features (e.g., cytogenetic,
molecular, presence of viral genomes, etc.).
B- and T-cell tumors are composed of cells derived from specific stages of their normal
differentiation pathways. The diagnosis and classification of these tumors relies heavily on
tests (e.g. immunohistochemistry & flow cytometry) that detect specific antigens (e.g., B-cell,
T-cell markers).
B- and T-cell tumors are composed of cells derived from specific stages of their normal
differentiation pathways. All lymphomas are derived from a single transformed cell and
thus are by definition monoclonal.
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■Non-Hodgkin lymphomas (NHL):
●Precursor B- and T-Cell Lymphoblastic Leukemia/Lymphoma: These are aggressive
tumors, composed of immature lymphocytes (lymphoblasts). They are microscopically
indistinguishable. Both pre-B- and pre-T-lymphoblastic lymphomas usually take on the
clinical appearance of an acute lymphoblastic leukemia (ALL) at some time during their
course. As a group, ALLs constitute 80% of childhood leukemia, peaking in incidence at age
of 4 years, with most of the cases being of pre-B-cell origin. The pre-T-cell tumors, which
initially present as thymic tumors, are most common in adolescent males of between 15 and
20 years of age.
●Small Lymphocytic Lymphoma/Chronic Lymphocytic Leukemia (SLL/ CLL): These
two disorders differ only in the extent of peripheral blood involvement.
If there is peripheral blood lymphocytosis > 4000 /µL, the patient is diagnosed with
chronic lymphocytic leukemia (CLL); if not, a diagnosis of small lymphocytic lymphoma
(SLL) is made.
Microscopic features: There is diffuse effacement of the lymph node architecture by
lymphoid cells. The predominant cells are compact, monomorphic, small, resting
lymphocytes with dark-staining round nuclei & scanty cytoplasm. There are, in
addition, foci of mitotically active larger cells; these aggregated to form proliferation
centers. The latter are pathognomonic for CLL/SLL. CLL/SLL is a neoplasm of mature B
cells expressing pan-B-cell markers such as CD19, CD20. In addition to the lymph nodes, the
bone marrow is involved in almost all cases; this is associated with peripheral absolute
lymphocytosis. Generalized lymphadenopathy and hepatosplenomegaly are present in > 50%
of cases. CLL/SLL tends to transform to more aggressive lymphoid malignancies.
●Follicular Lymphomas: The tumor likely arises from germinal center B cells. The bone
marrow is almost always involved at the time of diagnosis.
Microscopic features: The native lymph node architecture is effaced by nodular (follicular)
aggregates of proliferating lymphoid cells.
Most commonly, the predominant neoplastic cells are "centrocyte-like" cells; these are
slightly larger than resting lymphocytes & have "cleaved" nuclei due to prominent
indentations and linear infoldings of the nuclear membrane.
The small, cleaved cells are mixed with variable numbers of larger "centroblast-like" cells
that have vesicular chromatin, several nucleoli, and moderate amounts of cytoplasm. These
tumors express the pan-B-cell markers.
The majority of tumors have a characteristic t(14;18) translocation. This translocation
fuses the BCL
2
gene to the IgH locus on chromosome 14 and leads to the inappropriate
expression of BCL
2
protein, which functions to prevent apoptosis.
●Diffuse Large B-Cell Lymphoma (DLBCL): this is the most important type of NHL
lymphoma in adults (50% of adult NHL).
Microscopic features: The nuclei of the neoplastic B cells are large (3-4 times the size of
resting lymphocytes).
There are two morphologic variants of DLBCLs:
A. Centroblastic: the cells have round, or cleaved nuclear contours & several distinct
nucleoli, and moderate amounts of pale cytoplasm.
B. Immunoblastic: the cells have a large round or multilobulated nucleus, one or two
centrally placed prominent nucleoli, and abundant cytoplasm.
The cells express pan-B-cell antigens.
Approximately 30% of tumors have a t(14;18) translocation involving the BCL
2
gene.
Such tumors may represent "transformed" follicular lymphomas.
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●Burkitt Lymphoma: is endemic in some parts of Africa and sporadic elsewhere. Both the
African and nonendemic forms are identical, although there are clinical and virologic
differences.
*In endemic areas: tumor cells in almost all patients carry EBV genome. In such endemic
areas concomitant (endemic) malaria (or other infections) impairs immune competence,
allowing sustained B-cell proliferation. Lymphoma cells emerge only when additional
mutations, such as the t(8;14) translocation. This involves MYC proto-oncogene on
chromosome 8. The latter becomes activated to MYC oncogene as a result of fusion with
the IgH gene on chromosomes 14. The net result is the dysregulation and overexpression
of the MYC protein.
*In nonendemic areas: 80% of tumors DO NOT harbor the EBV genome, but all tumors
possess the specific t(8 ; 14) translocation. This observation suggests that, although non-
African Burkitt lymphomas are triggered by mechanisms other than EBV, they develop
cancer by similar pathways.
Microscopic features: The tumor cells are uniform and intermediate in size with round nuclei
containing 2 to 5 prominent nucleoli. There is a moderate amount of basophilic cytoplasm,
which on cytological smears often contains small lipid vacuoles.
A high mitotic rate is very characteristic of this tumor, as is cell death. The latter accounts
for the presence of numerous tissue macrophages containing ingested nuclear debris.
Because these benign macrophages with their pale cytoplasm are scattered within a blue
background (of lymphoma cells) a "starry sky" pattern is thus created.
These B-cell tumors express surface IgM, κ or λ light chain, and the pan-B-cell markers.
●Mantle Cell Lymphomas: are composed of B cells that resemble cells in the mantle zone of
normal lymphoid follicles. They involve lymph nodes in a diffuse or vaguely nodular pattern.
The tumor cells are usually slightly larger than normal lymphocytes and have an irregular
nucleus. The bone marrow is involved in the majority of cases, and about 20% of patients
have peripheral blood involvement. One unexplained but characteristic tendency is the
frequent involvement of the gastrointestinal tract.
■Hodgkin Lymphoma (HL): encompasses a distinctive group of neoplasms that arise almost
invariably in a single lymph node or chain of lymph nodes and spread characteristically in a
stepwise fashion to the anatomically contiguous nodes. HL accounts for 30% of all
lymphomas.
Classification: HL has been classified into:
1. Classical HLs.
2. Nodular lymphocyte predominant HL.
►Classical Hodgkin's lymphoma are subclassified into 4 pathologic subtypes based upon
Reed-Sternberg cell morphology and the composition of the reactive cell infiltrate seen in the
lymph node biopsy specimen.
Microscopic features: The characteristic microscopic feature is the presence of distinctive
neoplastic giant cells called Reed-Sternberg (RS) cells in a reactive background of
prominent inflammatory response composed of a variable number of lymphocytes,
macrophages, eosinophils, plasma cells and fibroblasts.
RS cell is a large cell (15-45 μm in diameter) with an enlarged bi- or multilobated nucleus,
prominent nucleoli, and abundant, usually slightly eosinophilic cytoplasm. Particularly
characteristic are cells with two mirror-image nuclei or nuclear lobes, each containing a large
acidophilic nucleolus surrounded by a distinctive clear zone; together they impart an owl-eye
appearance. The nuclear membrane is thick.
Also present are large cells of a similar appearance but with a single round nucleus and a very
large nucleolus, designated mononuclear Hodgkin cells.
Immunophenotype:
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Hodgkin and RS cells are CD30 positive in nearly all cases, and CD15 positive in most.
In 30-40% of cases CD20 may be detected, usually of varied intensity and in a minority
of Hodgkin and RS cells.
►Classical HL: Under this heading are the following subtypes:
●Nodular Sclerosis Hodgkin Lymphoma: is the most common form. It is equally frequent
in men and women and has a striking tendency to involve the lower cervical, supraclavicular,
and mediastinal lymph nodes. Most of the patients are adolescents or young adults, and the
overall prognosis is excellent.
It is characterized microscopically by the presence of a particular variant of the RS cell,
the “lacunar cell”. This cell is large and has a single multilobated nucleus with multiple
small nucleoli and an abundant, pale-staining cytoplasm. In formalin-fixed tissue, the
cytoplasm often retracts, giving rise to the appearance of cells lying in empty spaces, or
lacunae. The presence of collagen bands that divide the lymphoid tissue into
circumscribed nodules. The cellular infiltrate may show varying proportions of
lymphocytes, eosinophils, histiocytes, and lacunar cells. Classic RS cells are infrequent.
Lacunar variants express CD15 & CD30.
●Mixed-Cellularity Hodgkin Lymphoma (MCHL): is the most common form of Hodgkin
lymphoma in patients older than the age of 50 years with a male predominance. Classic RS
cells are plentiful within a distinctive mixed cellular infiltrate, which includes small
lymphocytes, eosinophils, plasma cells, and benign histiocytes. Compared with the other
subtypes, patients with mixed cellularity have more disseminated disease and systemic
manifestations.
●Lymphocyte-depleted Hodgkin disease (LDHL) (< 1% of cases): it is characterized by the
presence of large numbers of RS that are often of bizarre morphology. Reactive cells are
infrequent and fibrosis may be extensive. It is associated with older age and HIV positive
status.
●Lymphocyte-rich HL (LRHL) (5% of cases): RS cells of the classic or lacunar type are
observed with a background infiltrate of B-lymphocytes.
►Nodular lymphocyte predominant HL (NLPHL):
NLPHL is a monoclonal B-cell tumor considered to be of germinal centre B- cell origin.
It is characterized by formation of poorly defined nodules (nodular) or (nodular &
diffuse) with proliferation of scattered large multilobated neoplastic cells ‘LP cells’ in an
inflammatory background, which contain a large number of small mature lymphocytes
admixed with a variable number of benign histiocytes. Classic RS cells are difficult to find.
Unlike classic RS cells, LP cells are: Positive for B-cell antigens (CD19, CD20, CD79a),
Negative for CD15 & CD30.
A diagnosis of NLPHD needs to be supported by immunohistochemical studies because it can
appear similar to lymphocyte-rich HL or even some non-Hodgkin lymphomas.
The histologic diagnosis of Hodgkin lymphoma rests on the definitive identification of
RS cells or their variants in the appropriate background of reactive cells.
Immunophenotyping plays an important adjunct role in helping to distinguish Hodgkin
lymphoma from reactive conditions and other forms of lymphoma.
Clinical Staging of Hodgkin (and Non-Hodgkin Lymphoma) (Ann Arbor Classification):
Stage I: involvement of a single lymph node region (I) or involvement of a single
extralymphatic organ or tissue (I
E
).
Stage II: involvement of two or more lymph node regions on the same side of the diaphragm
alone (II) or with involvement of limited contiguous extralymphatic organs or tissue (II
E
).
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Stage III: involvement of lymph node regions on both sides of the diaphragm (III), which
may include the spleen (III
S
), limited contiguous extralymphatic organ or site (III
E
), or both
(III
ES
).
Stage IV: multiple or disseminated foci of involvement of one or more extralymphatic organs
or tissues with or without lymphatic involvement.
All stages are further divided on the basis of the absence (A) or presence (B) of the following
systemic symptoms: significant fever, night sweats, unexplained loss of more than 10% of
normal body weight.
Etiology and Pathogenesis of HL:
Hodgkin lymphoma (whether classical HL or NLPHL) is a neoplasm arising from germinal
center B cells. All RS cell and its variants in Classical HL show somatic hypermutation
that leads to Ig gene rearrangements and preclude expression of B cell markers, except
in NLPHL variant (ongoing somatic hypermutation).
Some cases are etiologically linked to EBV and incidence is increased in HIV-positive
individuals. The EBV genome is present in the RS cells in up to 70% of cases of the mixed-
cellularity type and a smaller fraction of the nodular sclerosis type. Thus, EBV infection is
likely to be a contributing step to the development of Hodgkin lymphoma, particularly the
mixed-cellularity type. It has been found that the RS cells in classical forms of Hodgkin
lymphoma, regardless of their EBV status, contain high levels of activated NF-κB, a
transcription factor that normally stimulates B-cell proliferation and protects B cells from
pro-apoptotic signals. Several EBV proteins that are known to activate NF-κB are expressed
in EBV-positive RS cells. Thus, hyperactivation of NF-κB may be a central event in the
genesis, growth, and survival of RS cells. The characteristic non-neoplastic, inflammatory-
cell infiltrate seems to result from a number of cytokines secreted by RS cells, including IL-5
(which attracts and activates non-neoplastic, inflammatory cell infiltrate & eosinophils).
These produce CD30 ligand which maintain the growth & survival of RS cells.
Spleen:
Splenomegaly: The spleen is frequently secondarily involved in a wide variety of systemic
diseases. In virtually all instances, the response of the spleen causes its enlargement
(splenomegaly). As an aid to diagnosis, splenomegaly is classified according to the degree of
its enlargement:
■Massive splenomegaly (weight more than 1000 gm):
●Chronic myeloproliferative disorders (chronic myeloid leukemia & primary myelofibrosis).
●Chronic lymphocytic leukemia.
●Hairy cell leukemia.
●Lymphomas.
●Malaria.
■Moderate splenomegaly (weight 500-1000 gm):
●Chronic congestive splenomegaly (portal hypertension or splenic vein obstruction).
●Acute leukemias.
●Hereditary spherocytosis.
●Thalassemia major.
●Autoimmune hemolytic anemia.
■Mild splenomegaly (weight <500 gm):
●Acute splenitis.
●Acute splenic congestion.
●Infectious mononucleosis.
●Miscellaneous acute febrile disorders, including septicemia, SLE, and intra-abdominal
infections.
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An enlarged spleen often removes excessive numbers of one or more of the formed
elements of blood, resulting in anemia, leukopenia, or thrombocytopenia. This is referred to
as hypersplenism, a state that can be associated with many of the diseases affecting the spleen
listed above. In addition, platelets are particularly susceptible to sequestration in the red pulp;
as a result, thrombocytopenia is more prevalent and severe in individuals with splenomegaly
than are anemia or neutropenia.
Thymus:
Thymic Hyperplasia: Hyperplasia of the thymus is often associated with the appearance of
lymphoid follicles, or germinal centers, within the medulla. These germinal centers contain
reactive B cells, which are normally present in only low numbers in the thymus. Thymic
follicular hyperplasia is present in most patients with myasthenia gravis and is sometimes also
found in other autoimmune diseases, such as SLE and rheumatoid arthritis.
Thymomas: These are tumors in which epithelial cells constitute the neoplastic element.
Scant or abundant precursor T cells (thymocytes) are present in these tumors, but these are
non-neoplastic.
Thymomas are classified as follows
■Benign or encapsulated (60-70%): cytologically and biologically benign.
■Malignant thymoma:
●Type I (20-25%): cytologically benign but biologically aggressive and capable of local
invasion and, rarely, distant spread.
●Type II (Thymic carcinoma) (5%): cytologically & biologically malignant.
Gross features: Thymomas are lobulated, firm, gray-white masses up to 15 to 20 cm in
longest dimension. Most appear encapsulated, but in 20% to 25% there is apparent penetration
of the capsule and infiltration of perithymic tissues and structures.
In thymic carcinomas metastases occur e.g. to the lungs.
Microscopic features: Almost all thymomas are made up of a mixture of epithelial cells and
a variable infiltrate of non-neoplastic thymocytes (T-lymphocytes).
Benign thymomas: show spindled or elongated epithelial cells that resemble those normally
populate the medulla. In other tumors the above cells are admixed with plump, rounder,
cortical-type epithelial cells; this pattern is referred to as a mixed thymoma.
Malignant Thymoma Type I: is a tumor that is cytologically bland but locally invasive.
They are composed of varying proportions of epithelial cells and reactive thymocytes; the
epithelial cells have abundant cytoplasm and rounded vesicular nuclei. They often form
palisades around blood vessels. Sometimes spindled epithelial cells are also present. The
critical distinguishing feature is the penetration of the capsule and the invasion of
surrounding structures.
Malignant Thymoma Type II (Thymic Carcinoma): they are malignant cytologically.
Most resemble either poorly or well-differentiated squamous cell carcinomas. The next most
common malignant pattern is undifferentiated lymphoepithelioma-like carcinoma, which is
composed of anaplastic cortical-type epithelial cells mixed with large numbers of benign
thymocytes.