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Hematology Pathophysiology: Benign Lymphoid Process (Al-Katib)
LYMPHATIC SYSTEM:

Primary Lymphoid Organs:
Bone marrow
Thymus
o Cortex
o Medulla

Secondary Lymphoid Organs:
Lymph nodes:
o Cortex (location of lymphoid follicles; B cell area)

Mantle Zone (site of memory B cells)

Germinal Center (site of B cell development)
o Paracortex (T cell area)
o Medullary cords
Spleen:
o Red pulp
o White pulp

Mantle Zone (site of memory B cells)

Germinal Center (site of B cell development)

T cell marginal area (PALS)
Tonsils
Intestinal lymphoid tissues

Lymph Circulation:
Lymph: clear fluid squeezed from blood vessels into interstitial spaces
Circulation: travels through lymph nodes carrying lymphocytes from one LN to another and eventually to the
vascular system
o Right Lymphatic Duct: drains lymph from upper extremities and chest into circulation
o Left Thoracic Duct: drains lymph from below the diaphragm into circulation

Main Functions of the Lymphatic System:
To collect and return interstitial fluid (including plasma protein) to the blood, thus maintaining fluid balance
To defend body against infectious agents (immune response)
To absorb lipids from the intestine and transport them to the blood
NORMAL LYMPHOCYTES:

Small Lymphocytes: most common type in normal blood
High N:C ratio
Round nucleus
Thin basophilic cytoplasm
Capable of ameboid movement

Large Lymphocytes: not very common in normal blood
Indented nucleus
More abundant cytoplasm, frequently with azurophilic granules (large granular lymphocytes/NK cells)

Functional Subsets of Lymphocytes:
B Cells: mature in bone marrow and give rise to plasma cells (humoral immunity)
T Cells: mature in thymus (cell mediated immunity)
NK Cells: subset of T cells with special function

Comparison of B and T Cells:
T Cells
B Cells
Origin
Thymus
Bone marrow
Tissue Distribution
Parafollicular areas of LN cortex
Germinal centers of LN (cortex), spleen,
Periarteriolar sheath in spleen
gut and respiratory tract
Subcapsular and medullary cords of LNs
Blood
80% of lymphocytes in blood (CD4>CD8)
20% of lymphocytes in blood
Membrane Receptor
TCR for Ag
BCR (Ig) for Ag
Function
CD8: CMI (intracellular organisms)
Humoral immunity (Ab production)
CD4: help for Ab production and CMI
Genes Rearranged
TCR alpha, beta, gamma, delta
IgH, IgKappa, IgLambda
Characteristic Surface Markers
CD1
CD2
CD3
CD4 or 8
CD5
CD6
CD7
HLA class I
HLA class II (when activated)
CD19
CD20
CD22
CD9 (pre-B cells)
CD10 (precursor B cells)
CD79a and b
HLA class I and II
B CELLS:

B Cell Differentiation:
Bone Marrow: differentiation from hematopoietic stem cell stage to mature B cell takes place in BM
o Antigen INDEPENDENT
o Mature B cell has IgM and IgD on its surface
Blood: mature B cells enter the blood to reach secondary lymphoid organs to finish their maturation
Secondary Lymphoid Organs: differentiation from mature B cell to plasma cell and memory cell
o Takes place in the GERMINAL CENTERS
o Antigen DEPENDENT
o Generally requires cooperation between T and B cells
o Once plasma cells are formed, they home back to the bone marrow where they remain until needed

B Cell Receptor:
Membrane bound Ig formed from 2 heavy chains and 2 light chains (covalently bound)
Ag binding unit is associated with CD79 heterodimer (a and b), which acts as a signal transducer
Surface Ig (BCR) is the classic marker of MATURE B CELLS*

Immunoglobulin Rearrangement:
Heavy chain rearranges first
Kappa light chain rearranges 2nd; if not successfully, lambda light chain will rearrange after

Immunoglobulin Structure and Function:
IgG: most abundant, serum and extracellular fluid, can cross placenta
IgA: dimer, found in serum and body secretions (mucosa associated)
IgM: large pentamer, only in the serum, very good at fixing complement
T CELLS:

T Cell Differentiation:
Thymus: produces immunocompetent T cells
o Huge numbers produced in the cortex (vast majority do not leave)

Cells begin as double negative (no CD4 or CD8) and then become double positive (CD4 + CD8)

Cells undergo positive and negative selection to become single positive (CD4 or CD*)
o Immunocompetent T cells leave the cortex via medullary postcapillary venules to enter the medulla

T Cell Receptor:
Consists of a number of components that form the CD3 complex
o 2 antigen binding chains (alpha and beta)
o Signal transduction proteins
Can only recognize Ag in the form of short peptides presented to them on HLA molecules
o CD8 cells interact with class I HLA; CD8 heterdimer interacts with alpha3 domain of class I protein

Functional Classes of T Cells:
Cytotoxic T Cells (CD8): recognizes HLA class I Ags (HLA-A, B and C)
Helper T Cells (CD4): recognizes HLA class II Ags (HLA-DR, DP and DQ)
NK Cells: cytotoxic T cells that lack TCR
o LGL morphology
o Cell Markers:

CD16+ (Fc receptor)

CD56+

CD57+
o Function to kill target cells with low HLA class I expression
o Leukemic transformation of these cells results in LGLL
COMPLEMENT SYSTEM:

Function: series of plasma proteins that can lyse bacteria or blood cells when activated
Also opsonize bacteria/cells to aid in phagocytosis

Mechanism:
C3 converted to C3b by C3 convertase
o C4b2b in classical pathway
o C3bBb in alternative pathway
C3b coats target cells leading to phagocytosis by PMNs and macrophages (have C3b receptors)
Completion of complement activation (generation of C9 and MAC) leads to cell lysis
o Generates phospholipase which punches holes in cell membrane
THE SPLEEN:

Structure:
Red Pulp: arteriolar cords and venous sinuses (~75% of the spleen)
o Monitors integrity of RBCs
White Pulp: lymphatic tissue surrounding central arterioles with prominent marginal zone

Function:
Control of red cell integrity (nuclear remnants, sideroblastic granules)
Immune functions (highly efficient for encapsulated bacteria in the blood)
Extramedullary hematopoiesis under pathological conditions

Hypersplenism:
Enlargement of the spleen (normal spleen is NOT palpable)
Reduction of at least 1 cell line in the blood in the presence of NORMAL BM function (due to sequesteration)
o As the spleen enlarges, it sequesters more and more blood cells in it

Causes of Splenomegaly:
Hematological:
o CML (may be massive)
o CLL
o Acute leukemia
o Lymphoma (may be massive)
o Primary myelofibrosis (may be massive)
o PV
o HCL
o Thalassemia major or intermedia (may be massive)
o Sickle cell anemia (before splenic infarction)
o Hemolytic anemias
o Megaloblastic anemias
Portal Hypertension:
o Cirrhosis
o Hepatic, portal, and splenic vein thrombosis
Storage Disease:
o Gaucher’s Disease (may be massive)
o Niemann-Pick
o Histiocytosis X
Systemic Diseases:
o Sarcoidosis
o Amyloidosis
o Collagen diseases (SLE, RA)
o Systemic mastocytosis
Infections:
o Acute: septicemia, bacterial endocarditis, typhoid, infectious mono
o Chronic: TB, brucellosis, syphilis, malaria, leishmaniasis, schistomiasis

Indications for Splenectomy:
Splenic rupture
Chronic immune thrombocytopenia
Some cases of hemolytic anemia (ie. hereditary spherocytosis, AIHA, thalassemia major or intermedia)
Chronic lymphocytic leukemia and lymphoma
Primary myelofibrosis

Important Points for Asplenic Patients:
Pneumococcal vaccine at least 2 WEEKS PRIOR to splenectomy
Annual influenza vaccine
H. flu and meningococcal vaccine if not previously done
LYMPHOCYTE DISORDERS:

Classification:
Benign (Reactive) Disorders:
o Lymphocytosis/Lymphadenopathy
o Lymphocyte Deficiency or Dysfunction
Malignant (Neoplastic) Disorders:
o Leukemias (acute and chronic)
o Lymphomas (HL an NHL)
o Immunoproliferative disorders

Assessment of the Lymphatic System for Detection of Disorders:
Physical Exam:
o Lymphadenopathy (localized or generalized)
o Splenomegaly
o Hepatomegaly
Imaging Studies:
o CAT scan
o PET scan
Blood Counts:
o Lymphocytosis
o Lymphopenia
o Abnormal morphology of lymphocytes
Bone Marrow Examination
Specialized Testing: of peripheral blood, BM or tissue biospsies
o Immunophenotyping (flow cytometry)
o Cytogenetics
o FISH
o Gene rearrangements (TCR, Ig)
LYMPHADENOPATHY

Definition: >1cm LN (exception is inguinal areas where >2cm is the cut off)
Generalized Lymphadenopathy: 3 or more non-contiguous areas of enlargement

Statistics:
Majority of causes (~80%) of lymphadenopathy are BENIGN (ie. infection)
The rest (~20%) are due to malignancy (lymphoma or metastatic carcinoma)

Lymph Node Hyperplasia (Benign):
Follicular Hyperplasia: hyperplastic germinal center with normal mixture of cells
o Variably sized lymphocytes
o Plasma cells
o Macrophages
o Dendritic reticular cells
Diffuse Hyperplasia: LN architecture diffusely effaced by sheets of small lymphocytes
o Also a few immunoblasts and macrophages
Sinus Hyperplasia: sinuses become distended and filled with histiocytes and some plasma cells
Mixed Hyperplasia: combination of follicular, sinus and diffuse patterns of hyperplasia

Characteristics of Lymphadenopathy:
Onset:
o Rapid (Days): the rule is INFECTION; very aggressive lymphoma is the exception
o Very Slow (Months to Years): low grade lymphoma or CLL
Pain/Tenderness:
o Yes: infection (less commonly very aggressive lymphoma)
o No: lymphoma
LN Consistency:
o Soft: benign


o Firm: NHL
o Rubbery Firm: HL (nodular sclerosing type)
o Woody Hard: metastatic carcinoma
Localized Lymphadenopathy Differential Diagnosis:
GOLDEN RULE: FOCUS ON THE AREA DRAINED BY THE ENLARGED LNs*
Local Infection:
o Pyogenic infection (ie. pharyngitis, dental abscess, otitis media, actinomyces)
o Viral infection
o Cat scratch fever
o Lymphogranuloma venereum
o TB
Lymphoma:
o HL
o NHL
o Carcinoma (secondary)
Generalized Lymphadenopathy Differential Diagnosis:
Infection (Systemic):
o Viral (ie. infectious mono, measles, rubella, hepatitis, HIV)
o Bacterial (ie. syphillus, brucellosis, TB, salmonella, bacterial endocarditis)
o Fungal (ie. histoplasmosis)
o Protozoal (ie. toxoplasmosis)
Non-Infectious Inflammatory Disease:
o Sarcoidosis
o RA
o SLE
o Other CT diseases
o Serum sickness
Miscellaneous:
o Sinus histiocytosis with massive lymphadenopathy
o Reaction to drugs and chemicals (ie. hydantoins, beryllium)
o Hyperthyroidism
Malignant:
o Leukemias (esp. ALL and CLL)
o Lymphomas (NHL, HL)
o Waldrenstrom macroglobulinemia
o Secondary carcinoma (RARE)
o Angiommunoblastic lymphadenopathy
LYMPHOCYTOSIS AND LYMPHOPENIA:

Lab Values:
Normal Lymphocyte Count:
o Birth-6 months: 5500-7300/uL
o Adults: 2500/uL
Lymphocytosis:
o Adults: >4000/uL
o Infants/Young Kids: >9000/uL
Relative Lymphocytosis: total lymphocyte count is normal but percentage is increased
Lymphopenia: <1500/uL

Causes of Lymphocytosis:
Absolute Lymphocytosis:
o Certain Acute Infections:

Bordetella pertussis

Infectious mono

Acute infectious lymphocytosis

Infectious hepatitis
o Certain Chronic Infections:

Brucellosis

TB


Toxoplasmosis

Secondary and congenital syphilis
o Lymphoid Malignancies:

CLL

ALL

Leukemic phase of lymphomas
o Distinguishing Between Benign and Malignant:

Clinical picture

Serology

Lymphocyte morphology

Immunophenotyping of lymphocytes by flow cytometry
 Polyclonal= benign
 Monoclonal= malignant
Relative Lymphocytosis: absolute count is normal but % is elevated
o Exanthems (after the initial stage; ie. mumps, German measles)
o Convalescent stage of acute infections
o Thryotoxicosis
o Most conditions associated with neutropenia
Causes of Lymphopenias:
Primary (Immunodeficieny Disorders):
o B Cell/Ab Deficiency: X-linked agammaglobulinemia
o T Cell Deficiency: DiGeorge Syndrome
o T and B Cell Mixed: SCID, Wiskott-Aldrich
Secondary:
o B Cell/Ab Deficiency: MM, anti-CD20 therapy
o T Cell Deficiency: AIDS, HL, NHL, drugs (ie. steroids)
o T and B Cell Mixed: CLL, post stem-cell transplant, chemotherapy, anti-CD52
INFECTIOUS MONONUCLEOSIS:

Basics:
Self limited acute febrile illness affecting YOUNG PEOPLE (15-40)
Caused by primary EBV infection (most of these are subclinical)
Results in expansion of T cells reactive against EBV infected B cells

Clinical Features:
Systemic Symptoms: malaise, headache, dry cough, stiff neck, fever, sore throat
Other Symtpoms:
o Lymphadenopathy (75% cervical)
o Oral/pharyngeal erythema with tonsillitis
o Skin rash
o Conjunctivitis
o Splenomegaly (>50% of cases)
o Hepatomegaly (15% of cases)
o Increased risk for HODGKIN LYMPHOMA later in life

Laboratory Findings:
Absolute lymphocytosis with ATYPICAL lymphocytes (large)
Positive heterophile Abs (against sheep or horse RBCs)
EBV IgM antibodies (acute phase- 2-3 weeks)
EBV IgG antibodies (later and life-long)

Treatment:
Supportive care
NO ANTIBIOTICS (rash in reaction to ampicilln)
Steroid for severe symptoms