Comprehensive Study Notes on the Hematopoietic System and Related Disorders

Overview and Learning Objectives

Source Material: Reading assignments include Robbins & Kumar, Basic Pathology, 11th ed, 2023, Chpt 10: 345-388 OR Robbins Basic Pathology, 10th edition, 2018, pp. 441-491.
Acknowledgments: Segments of the lecture were provided by Dr. Carl O’Hara, Professor of Pathology.
Intended Learning Outcomes: * Describe normal elements of bone marrow and peripheral blood. * List pathogenic mechanisms of iron deficiency and megaloblastic anemia. * Describe morphologic features of sickle cell anemia. * Discuss pathogenesis of Disseminated Intravascular Coagulation (DIC). * List two causes for aplastic anemia. * Discuss the division of leukemias into acute and chronic types (based on presentation/outcome) and myeloid or lymphoid cell lineages. * Develop a framework for malignant lymphoma classification. * Discuss how Hodgkin lymphoma differs morphologically, phenotypically, and clinically from other lymphomas. * Explain plasma cell disorders and morphology. * Understand complications of myelodysplastic syndrome. * Describe the pathogenesis of von Willebrand’s disease.
Scope of Hematopoietic System: Includes red cells, white cells, platelets, coagulation factors, lymph nodes, spleen, bone marrow, thymus, and specific blood vessel functions.
Challenges in Study: The system is complex due to its "fluid nature." Elements are not solid or confined to one anatomic region. While this allows for easier identification of certain diseases (like chronic leukemias), it complicates disease spread and makes traditional staging systems difficult to apply.

Red Blood Cell Disorders: Anemia

Definition: Anemia is defined as any reduction in the oxygen-carrying capacity of the blood.
Primary Causes: * Reduction in red blood cell (RBC) numbers. * Increased red cell destruction. * Functional defects of hemoglobins. * Coexisting conditions such as anemia of chronic disease.

Iron-Deficiency Anemia

General Characteristics: Most common form of nutritional deficiency.
Total Body Iron Distribution: * Women: Total of $2\,gm$ . * Men: Total of $6\,gm$ . * Hemoglobin: Contains $80\,\%$ of total body iron ( $Fe$ ). * Storage Pool: Stored as ferritin and hemosiderin. * Transportation: Carried via transferrin.
Normal Serum Iron ( $Fe$ ) Levels: * Men: $120\,\mu g/dl$ . * Women: $100\,\mu g/dl$ .
Iron Homeostasis Mechanisms: * Salvaging: Most iron required for the erythron is salvaged from the turnover of senescent (old) red cells. * Regulation: Balance is regulated primarily via absorption of dietary iron. * Western Diet Average: $10-20\,mg$ of iron (both heme and inorganic). * Daily Loss: $1-2\,mg/day$ via shedding of mucosal and skin cells. * Absorption Rate: Approximately $20\,\%$ of heme iron is absorbable; a typical diet is sufficient to cover daily loss. * Primary Absorption Site: The duodenum. * Fate of Absorbed Iron: Some binds immediately to transferrin for transport; the remainder binds to ferritin for storage.
Etiology/Causes of Iron Deficiency: * Low Dietary Intake: Rare in the United States. * Malabsorption: Conditions like sprue (inflammation of the duodenum leading to loss of villi). * Increased Demand: Pregnancy and infancy. * Chronic Blood Loss: The most common cause in the Western world. * Gastrointestinal (GI) Tract: Ulcers or tumors. * Uterus: Dysfunctional uterine bleeding.
Clinical Scenarios and Workup Contexts: * 60-year-old with occult blood in stools: Evaluate for colon cancer. * 35-year-old premenopausal woman: Consider excessive menstrual bleeding. * 2-year-old child: Evaluate diet and iron supplementation in formula. * Developing countries: Consider hookworm infestation.
General Symptoms: Weakness, lethargy, and tiredness.
Diagnostic Findings: Microcytic hypochromic anemia.

Megaloblastic Anemia: Vitamin B12 and Pernicious Anemia

Definition: A condition where red blood cells are larger than normal (macrocytic) associated with a hypercellular bone marrow. Often results from dietary deficiency of Vitamin $B_{12}$ or folic acid.
Pernicious Anemia (PA) Definition: A megaloblastic anemia associated with atrophic gastritis, complete achlorhydria, and malabsorption of Vitamin $B_{12}$ due to intrinsic factor ( $IF$ ) deficiency.
Properties of Vitamin $B_{12}$ (Cobalamin): * Complex organometallic compound containing a corrin ring and cobalt. * Source: Microorganisms are the ultimate source in the food chain. * Dietary Requirement: $2-3\,\mu g$ (milli-units in transcript note, likely micrograms) mainly from animal products; plants and vegetables contain very little. * Body Stores: Approximately $2-3\,grams$ in a normal adult.
Absorption Mechanism: 1. Salivary glands produce "R-binders." 2. Gastric Intrinsic Factor ( $IF$ ) is secreted by parietal cells. 3. $IF$ receptors are located on the cells lining the ileum (site of absorption).
Biochemical Reactions Requiring Vitamin $B_{12}$ : 1. Methyltransferase: Conversion of homocysteine to methionine (essential for generating $5, 10, \text{methylene } FH_4$ for DNA synthesis). 2. Isomerization: Conversion of methylmalonyl coenzyme A to succinyl coenzyme A.
Gastric Pathology in PA: * Anatomy: Fundus contains chief cells (enzymes) and parietal cells (secrete $HCl$ and $IF$ ). * Autoantibodies: * $90\,\%$ patients have anti-parietal cell antibodies. * $75\,\%$ patients have anti- $IF$ antibodies (blocking or binding types). * $50\,\%$ patients have antibodies to thyroid tissue. * $30\,\%$ of chronic thyroiditis (Hashimoto's) patients have anti-parietal cell antibodies and a high incidence of PA. * Epidemiology: Most prevalent in Northern Europeans; occurs in all races. More common in males than females, usually in the $50-80$ -year-old age group.
Systemic Manifestations: * Gastrointestinal: Atrophic glossitis (shiny, glazed, depapillated "beefy" tongue in $50\,\%$ of patients); Atrophic gastritis (chronic mucosal inflammation by lymphocytes/plasma cells, gland atrophy, and intestinal metaplasia involving goblet and Paneth cells). * Hematopoietic: Hypercellular bone marrow, erythroid hyperplasia, megaloblasts, nuclear/cytoplasmic dyssynchrony, giant metamyelocytes. Peripheral smear shows macroovalocytes ( $MCV > 115\,fl$ ), hypersegmented neutrophils, and giant platelets. * Neurologic (Subacute Combined System Disease): Symmetric numbness, tingling of extremities, impaired vibratory/position sense, and spastic ataxia. Caused by degeneration of the posterior and lateral columns of the spinal cord. Found in $75\,\%$ of untreated cases; can occur without anemia.
Laboratory and Diagnostic Tests: * Serum $B_{12}$ : Radioisotope dilution assay. Normal range is $150-650\,pg/ml$ . Clinical deficiency begins below $100\,pg/ml$ . * Schilling Test: Measures absorption of labeled Vitamin $B_{12}$ ( $Co^{57}$ ). Normal excretion in $24$ -hour urine is $10-20\,\%$ . If low, the test is repeated with exogenous $IF$ . Increased excretion after $IF$ suggests Pernicious Anemia.
Therapy: Lifelong Vitamin $B_{12}$ . Initial dose $1000\,\mu g$ IM weekly for several weeks, then monthly. Reticulocyte count rises on the third day. Neurologic improvement may take $6-12$ months; subsequent dysfunction is permanent.

Megaloblastic Anemia: Folate Deficiency

Function of Folic Acid (Tetrahydrofolate, $FH_4$ ): Acts as an intermediate for transferring one-carbon (methyl) units for: 1. Synthesis of purines. 2. Synthesis of methionine from homocysteine. 3. Synthesis of deoxythymidylate monophosphate (DNA synthesis).
Comparison to $B_{12}$ Deficiency: Both result in megaloblastosis due to impaired DNA synthesis. However, folate deficiency does not cause neurological defects.
Dietary Factors: * Requirement: $50-200\,mg/day$ (likely micrograms in standard medical context, transcript notes mg). Sources: Green vegetables, fruits, liver. * Body Reserves: Modest; deficiency can arise within months on a poor diet. * Sensitivity: Folates are destroyed by heat/boiling (overcooked foods). * Absorption: Occurs in the proximal jejunum.
Major Causes: 1. Decreased Intake: Inadequate diet (alcoholism, indigent status, elderly) or malabsorption (sprue, lymphoma, drugs like Dilantin). 2. Increased Requirement: Pregnancy, infancy, malignancy. 3. Impaired Use: Folic acid antagonists (Methotrexate, Cyclophosphamide) which inhibit dihydrofolate reductase.

Hemoglobinopathies and Hemolytic Anemia

Sickle Cell Anemia: * Pathogenesis: A mutation substituting valine for glutamic acid at position 6 of the beta-globin chain. * Mechanism: Causes hemoglobin to gelate or crystallize, leading to the characteristic "sickle" shape. Result is increased hemolysis and decreased oxygen-carrying capacity.
Hemolytic Anemia Etiology: * Altered RBC Shape: Example: Hereditary spherocytosis. * Immune Mechanisms: Example: $Rh$ incompatibility. * Mechanical Trauma: Example: Disseminated Intravascular Coagulation (DIC). Fibrin strands in circulation trap and cut red cell membranes.

Aplastic Anemia

Definition: A defect in the bone marrow stem cell.
Clinical Presentation: Pancytopenia (reduction in all marrow-derived cell elements).
Pathogenesis: Primarily considered autoimmune in origin. Can be precipitated by an "idiosyncratic" response to drugs or viral infections.

Lymph Nodes and Lymphoid Tissues: Structure and Immunology

Lymphatic System: An interconnecting system of vascular conduits with nodes widely distributed. Nodes range from a few millimeters to $1-2\,cm$ .
Aggregations: Cervical (neck), axilla, groin (clinically detectable); mediastinum and periaortic (require imaging).
Structural Compartments: * Cortex: Outermost portion. Contains secondary follicles consisting of a central germinal center surrounded by a mantle zone. * Germinal Centers: Composed of follicular center cells (small/large cleaved and non-cleaved). "Cleaved" refers to nuclear infolding. Scaffolded by follicular dendritic reticulum cells (antigen-presenting cells for B cells). * Mantle Zone: Composed of small round to slightly irregular B cells in early activation. * Paracortex: Area between follicles. Predominantly small lymphocytes with larger transformed immunoblasts. Contains dendritic reticulum cells (antigen-presenting cells). * Sinuses: Vascular-like channels lined by phagocytic histiocytes. Lymph enter via afferent lymphatics, into subcapsular sinuses, and exits via efferent lymphatics. * Medullary Cords: Inner medulla. Predominantly contains plasma cells and vascular sinusoids with histiocytes.
Immunology and Antigens: * Humoral Immunity: Mediated by B cells (derived from Bursa of Fabricius). B cell antigens: $CD19, CD20, CD10$ . * Cell-Mediated Immunity: Mediated by T cells (derived from Thymus). T cell antigens: $CD2, CD3, CD4, CD5, CD7, CD8$ . * Note: Division is not absolute; some B cells are in the paracortex, and T cells are in secondary follicles.

Lymphoproliferative Disorders: Non-Hodgkin Lymphoma (NHL)

General: Neoplastic transformation of lymphoid cells. History involves categorizing morphologic subtypes for prognostic outcome.
Historical Classifications: * Henry Rappaport (mid-1950s): Based on growth pattern (nodular vs. diffuse) and cell size/contours. Small cells and nodular patterns had better prognoses. (Incorrectly identified large cells as histiocytes). * Lukes, Collins, and Lennert (mid-1970s): Asserted that lymphomas should be divided by lineage (B or T cell). Observed that malignant cells correspond to specific compartments in the normal lymph node maturation sequence.
Clonal Expansions: Malignant lymphomas represent discrete stages of B cell maturation (from small round B cell to plasma cell). * Small lymphocytic type/Chronic Lymphocytic Leukemia (CLL): Malignancy of small round regular circulating lymphocytes. * Follicular Lymphomas: Malignancy of germinal center cells (cleaved/non-cleaved). Can be nodular or diffuse. * Burkitt Lymphoma: Malignancy of germinal center small non-cleaved cells. * Multiple Myeloma/Plasmacytoma: Malignancy of plasma cells.
Statistics: $80-85\,\%$ of NHL in the U.S. are B-cell type; $15\,\%$ are T-cell type.
Leukemia/Lymphoma Overlap: Many entities are distinct, but overlap exists (e.g., solid tissue lymphoma with concomitant leukemia like CLL/SLL).
WHO Classification (5 Broad Categories): 1. Precursor B-cell neoplasms (immature B). 2. Peripheral B-cell neoplasms (mature B). 3. Precursor T-cell neoplasms (immature T). 4. Peripheral T-cell and NK cell neoplasms (mature T/NK). 5. Hodgkin lymphoma.
Diagnostic Tools: Immunophenotypic studies (surface antigens, kappa/lambda light chain monoclonality) and molecular genetic studies (DNA rearrangement for monoclonality; translocations like $t(8;14)$ in Burkitt or $t(14;18)$ in follicular).

Hodgkin Lymphoma

Definition: A lymphoid malignancy composed of Reed-Sternberg (RS) cells in a mixed inflammatory background.
Cell of Origin: Recent evidence supports a B lymphoid origin for the RS cell.
Histology: * Classical RS Cells: Large bi-nucleated cells with prominent eosinophilic centrally located nucleoli surrounded by a halo; abundant cytoplasm. * RS Variants (Hodgkin Cells): Mononuclear, multinucleated, lacunar cells, and L/H (popcorn) cells. * Background Component: Usually comprise > $95\,\%$ of the population. Includes small lymphoid cells, histiocytes, plasma cells, eosinophils, and neutrophils. * Fibrosis: Ranges from negligible to marked band-like collagen deposition.
Rye Classification: 1. Lymphocyte Predominance (LP): Abundant lymphocytes, rare RS cells. 2. Nodular Sclerosis (NS). 3. Mixed Cellularity (MC). 4. Lymphocyte Depletion (LD): Sparse lymphocytes, increased RS cells.

Neoplasms of the Bone Marrow and Blood: Leukemias

Normal Hematopoiesis: * Stem cells originate in yolk sac blood islands, migrate to liver during gestation, and to bone marrow at birth. * Cell Lineages from Pluripotent Stem Cells: Erythroid (RBCs), Myeloid (WBCs: neutrophils, eosinophils, basophils, monocytes), Megakaryocytes (platelets), and Lymphocytes (B cells, T cells, plasma cells).
Leukemia ("White Blood"): Circulating abnormal cells in the blood. * Acute Leukemia: Circulating blasts ( $>30\,\%$ marrow cellularity). * Chronic Leukemia: Circulating mature cells.
Acute Myelogenous Leukemia (AML): * Morphology: Myeloblasts with large eccentric nuclei, delicate chromatin, $2-4$ nucleoli. May contain Auer rods (diagnostic). * Antigens: $CD13, CD33$ . Myeloperoxidase positive. * Clinical: Abrupt onset, fatigue, fever, bruising/bleeding due to anemia, leukopenia, and thrombocytopenia. * Epidemiology: Primarily adults ( $15-39$ years); $20\,\%$ of childhood leukemia. * Prognosis: $60\,\%$ achieve remission; $15-30\,\%$ remain disease-free at $5$ years.
Acute Lymphoblastic Leukemia (ALL): * Morphology: Lymphoblasts with round nuclei, condensed chromatin, scant agranular cytoplasm. * Markers: TdT (terminal deoxy-transferase) is very useful. * Subtypes: $85\,\%$ are pre-B cell type (childhood). T-cell type occurs in adolescent males (often with mediastinal mass). * Clinical: Peak age is $4$ . More common lymph node, spleen, liver, and CNS involvement compared to AML. * Prognosis: Aggressive chemotherapy leads to $90\,\%$ remission; $2/3$ are cured.
Chronic Myelogenous Leukemia (CML): * Morphology: Marrow hypercellularity with all stages of myeloid maturation (especially myelocytes, metamyelocytes, neutrophils). * Genetics: Philadelphia chromosome ( $Ph^1$ ); $t(9;22)$ involving $bcr$ (chr 9) and $abl$ (chr 22). * Presentation: Splenomegaly ( $2,000-3,000\,grams$ ), weight loss, LUQ dragging sensation. * Clinical Course: Median survival $3\,years$ . Evolves into "blast crisis" ( $70\,\%$ AML, $30\,\%$ ALL). * Treatment: Allogeneic bone marrow transplantation offers the best hope.

Plasma Cell Neoplasms: Multiple Myeloma

General: Proliferation of a B-cell clone secreting a homogeneous immunoglobulin or fragment (monoclonal gammopathy).
Markers: * M Component: The monoclonal immunoglobulin spike on Serum Protein Electrophoresis (SPEP). * Bence Jones Proteins: Free light chains excreted in the urine.
Pathology of Multiple Myeloma (MM): * Skeletal Involvement: Axial skeleton (vertebra $66\,\%$ , skull $41\,\%$ , pelvis $28\,\%$ , femur $24\,\%$ ). "Punched out" radiographic defects. * Marrow: Plasma cells comprising > $30\,\%$ of cellularity. * Ig Types: $IgG$ ( $55\,\%$ ), $IgA$ ( $25\,\%$ ); rare $IgM, IgD, IgE$ .
Clinical Features: * Hypercalcemia: Due to bone resorption; causes confusion and weakness. * Rouleaux Formation: High Ig causes RBCs to stick together. * Infections: Suppression of humoral immunity (Strep, Staph, E. coli). * Renal Insufficiency: $50\,\%$ of cases due to Bence Jones protein toxicity. * Amyloidosis: $10-15\,\%$ of cases.
Prognosis: Survival $6-12\,months$ .

Myelodysplastic Syndromes (MDS)

Definition: Previously termed "Preleukemia."
Characteristics: Group of disorders involving refractory cytopenias in older patients. Identified by chromosomal abnormalities. Eventually progress into Acute Myelogenous Leukemia (AML).

Bleeding and Clotting Disorders

Disseminated Intravascular Coagulation (DIC): * Acquired bleeding disorder where the coagulation cascade is activated, forming microthrombi. * Causes: Sepsis, malignancy, major trauma, obstetric complications. * Complications: One cause of hemolytic anemia; often fatal.
Von Willebrand’s Disease: * Type 1: Most common form; autosomal dominant inheritance. * Pathogenesis: Defective or deficient levels of von Willebrand factor ( $vWF$ ). * Presentation: Bleeding from mucus membranes and menorrhagia in women. Affects both the coagulation system and normal platelet function.