Year 3: Haematology and Blood Transfusion OmpathStudy

MOUNT KENYA UNIVERSITY SCHOOL OF MEDICINE — DEPARTMENT OF PATHOLOGY Course: Human Pathology — Year Three Unit: Haematology and Blood Transfusion Pathology Crash Revision Notes — Semester Two (Sem 1) Compiled for exam prep, June 2026 SEMESTER TWO — COMPLETE NOTES 1. Introduction to Haematology Study of blood, blood-forming organs (marrow, lymphoid tissue, spleen), and their diseases. Blood = plasma (~55%) + cells (~45%): RBCs, WBCs, platelets. Sites of haemopoiesis across life (must-know sequence): Mesoblastic stage — yolk sac (weeks 2–10 gestation) Hepatic stage — liver/spleen (6 weeks–7 months gestation) Medullary stage — bone marrow (from ~5 months gestation; sole site by birth) Adult red (active) marrow confined to: vertebrae, sternum, ribs, skull, pelvis, proximal femur/humerus. Yellow marrow (fatty, inactive) can revert to red marrow under stress (e.g. severe chronic anaemia). Essay angle : "Describe the sites of haemopoiesis from foetal life to adulthood" is a classic short essay — use the 3-stage sequence as your skeleton. 2. Haemopoiesis Process forming/maturing all blood cells from one ancestor: pluripotent haemopoietic stem cell (HSC) , capable of self-renewal and differentiation. Splits into two lineages: Myeloid → erythrocytes, granulocytes (neutrophils, eosinophils, basophils), monocytes, megakaryocytes/platelets Lymphoid → T cells, B cells, NK cells Growth factors (must-know matching for MCQs): Erythropoietin (EPO) → RBCs (renal origin, hypoxia-triggered) Thrombopoietin → platelets (mainly hepatic origin) G-CSF / GM-CSF → granulocytes Interleukins (IL-3, IL-7…) → early progenitors Bone marrow stroma (stromal cells, fat, sinusoids) physically/chemically supports developing cells — "the soil the seed grows in." 3. Erythrocytes Biconcave, anucleate discs, ~7–8 µm diameter, lifespan ~120 days . Function: O2/CO2 transport via haemoglobin; biconcave shape maximises surface area and flexibility for capillary passage. Maturation sequence (must-know, often asked to list in order): Proerythroblast → Basophilic erythroblast → Polychromatic erythroblast → Orthochromatic erythroblast → Reticulocyte → Mature erythrocyte Nucleus extruded at the orthochromatic stage — why mature RBCs are anucleate. Reticulocytes retain residual RNA (bluish/polychromatic tint on stain), circulate ~1–2 days before full maturity — reticulocyte count is a marker of marrow erythropoietic activity. Production driven by EPO; destruction mainly extravascular in the spleen — Hb broken down to globin (recycled to amino acids) and heme (→ iron recycled, protoporphyrin → bilirubin → excreted, hence jaundice if haemolysis is excessive). Red cell indices (must-know definitions — common MCQ stem): MCV — mean cell volume (average size) MCH — mean cell haemoglobin (average Hb mass per cell) MCHC — mean cell Hb concentration (Hb relative to cell volume) Haematocrit/PCV — % of blood volume occupied by RBCs 4. Haemoglobin Tetramer of 4 globin chains (adult: 2 alpha + 2 beta), each carrying one heme group with central Fe²⁺ that binds O2 reversibly. Loads O2 in lungs (high pO2), unloads in tissues (lower pO2, higher CO2/H⁺ — Bohr effect shifts the O2-Hb dissociation curve right, favouring unloading). Haemoglobin types (must-know chain composition — classic matching question): HbA (α2β2) — major adult Hb (~97%) HbA2 (α2δ2) — minor adult component (~2%) HbF (α2γ2) — dominant foetal Hb, falls sharply after birth (residual <1% in adults) Synthesis requires: adequate iron supply, normal globin genes (alpha cluster — chromosome 16; beta cluster — chromosome 11), and protoporphyrin (heme synthesis pathway). Abnormal haemoglobins (high-yield for both MCQ and essay): HbS (sickle cell disease) — single point mutation, beta chain: glutamic acid → valine at position 6; causes Hb polymerisation under deoxygenation → sickling, vaso-occlusion, haemolysis Thalassaemias — reduced/absent globin chain synthesis (alpha-thalassaemia = alpha gene defect; beta-thalassaemia = beta gene defect) → chain imbalance → ineffective erythropoiesis + haemolysis 5. Anaemia Definition: Hb/red cell mass below normal reference range for age and sex → reduced O2-carrying capacity. Classify by mechanism (essay skeleton #1): Decreased production — iron deficiency, megaloblastic anaemia (B12/folate deficiency), aplastic anaemia, anaemia of chronic disease, marrow infiltration (leukaemia, metastases) Increased destruction (haemolytic) — hereditary (sickle cell disease, thalassaemia, hereditary spherocytosis, G6PD deficiency) or acquired (autoimmune haemolytic anaemia, malaria, microangiopathic haemolytic anaemia/DIC) Blood loss — acute (trauma/haemorrhage) or chronic (GI bleed, menorrhagia) Classify by morphology using MCV (essay skeleton #2 — the one examiners love most): Microcytic (MCV <80fl) — iron deficiency, thalassaemia, anaemia of chronic disease, sideroblastic anaemia Normocytic (MCV 80–100fl) — acute blood loss, anaemia of chronic disease, early haemolytic anaemia, aplastic anae