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⭐⭐⭐ Q1: Define Erythropoiesis. Stages involved and factors affecting it?

Definition

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Site of Erythropoiesis - Changes with Age

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Stages of Erythropoiesis

1. Proerythroblast (Pronormoblast)

• Largest cell in the series (~20 µm)
• Nucleus: large, prominent nucleoli, fine chromatin
• Cytoplasm: deeply basophilic (rich in ribosomes for Hb synthesis)
• No hemoglobin yet

2. Early Normoblast (Basophilic Erythroblast)

• Slightly smaller than proerythroblast
• Nucleus: chromatin becomes coarser, nucleoli disappear
• Cytoplasm: still deeply basophilic
• Hemoglobin synthesis begins

3. Intermediate Normoblast (Polychromatophilic Erythroblast)

• Cell size decreases further (~15 µm)
• Nucleus: clumped chromatin (clock-face pattern)
• Cytoplasm: Mixed - both basophilic and eosinophilic (polychromatophilia) due to increasing Hb
• Active mitosis occurs here

4. Late Normoblast (Orthochromatic Erythroblast)

• Cell is smaller (~10 µm)
• Nucleus: very condensed, pyknotic (dark, small)
• Cytoplasm: predominantly eosinophilic (Hb accumulation nearly complete)
• No more mitosis occurs; nucleus is extruded

5. Reticulocyte

• Nucleus has been extruded
• Cell is ~8-9 µm
• Still contains remnant organelles (ribosomes, mitochondria) - seen as a reticulum with special stain (brilliant cresyl blue)
• Spends 2-3 days in marrow, then released into blood
• Normal reticulocyte count in blood: 0.5-2.5% of RBCs
• Elevated reticulocyte count = sign of active erythropoiesis (e.g., after blood loss)

6. Mature Erythrocyte (RBC)

• Biconcave disc, 7.2 µm diameter
• No nucleus, no organelles
• Filled entirely with hemoglobin
• Lifespan: 120 days

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Factors Affecting Erythropoiesis

A. Factors Stimulating Erythropoiesis

• The most important regulator
• A glycoprotein hormone produced 85-90% by peritubular interstitial cells of the kidney; 10-15% by liver
• Stimulus: Hypoxia (low O₂ tension in kidney)
• Action: Acts on committed progenitor cell (CFU-E), promotes differentiation and proliferation, inhibits apoptosis of erythroid precursors
• Result: Increased RBC production within 5 days

• Iron: For heme synthesis (Fe²⁺ + protoporphyrin IX = heme). Absorbed in duodenum as Fe²⁺. Deficiency leads to microcytic hypochromic anemia.
• Vitamin B12 (Cyanocobalamin): Required for DNA synthesis (thymidine synthesis via folate cycle). Deficiency causes megaloblastic anemia.
• Folic Acid (Vitamin B9): Also needed for DNA synthesis (tetrahydrofolate pathway). Deficiency also causes megaloblastic anemia.
• Proteins/Amino acids: For globin chain synthesis.
• Vitamin C: Aids Fe absorption (reduces Fe³⁺ to Fe²⁺) and protects RBCs from oxidative damage.
• Vitamin B6: For heme synthesis (ALA synthase reaction).
• Copper: Cofactor for ceruloplasmin, which oxidizes Fe²⁺ to Fe³⁺ for transferrin binding.

• Stimulate EPO production from kidney
• Also directly stimulate bone marrow
• Explains why men have higher RBC count than women

• Mild stimulatory effects on erythropoiesis

B. Factors Inhibiting Erythropoiesis

• Polycythemia (excess RBCs suppress EPO)
• Chronic kidney disease (reduced EPO production)
• Chronic inflammation (hepcidin inhibits iron availability)

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Vitamins and Role in RBC Maturation

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⭐⭐⭐ Q2: Define Haemostasis? Intrinsic and Extrinsic Pathway of Coagulation?

Definition

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Mechanisms of Haemostasis (4 Steps)

Step 1: Vascular Spasm (Vasoconstriction)

• Immediately after injury, the damaged vessel constricts due to:
  • Local myogenic reflex
  • Local autocoids (thromboxane A₂, serotonin from platelets)
  • Neural reflexes
• Reduces blood flow to the injured area
• Duration: few seconds to minutes

Step 2: Formation of Platelet Plug (Primary Haemostasis)

• Platelet adhesion: Exposed subendothelial collagen + Von Willebrand factor (vWF) → platelets adhere via GP Ib receptor
• Platelet activation: Activated platelets release ADP, thromboxane A₂, serotonin from granules
• Platelet aggregation: ADP and TXA₂ recruit more platelets; they bind each other via fibrinogen + GP IIb/IIIa receptors
• Forms a loose primary platelet plug
• Adequate for small vessel injuries (tested by bleeding time: 2-5 minutes)

Step 3: Blood Coagulation (Secondary Haemostasis)

• Converts the loose platelet plug into a firm, stable clot
• Involves the coagulation cascade
• Results in fibrin formation

Step 4: Clot Retraction and Fibrinolysis

• Clot retraction (by platelet actin/myosin) seals the vessel
• Fibrinolysis later dissolves the clot as healing occurs (plasmin cleaves fibrin)

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Blood Coagulation Cascade

General Principles

• Most clotting factors are serine proteases (zymogens) activated by proteolytic cleavage
• They act as an amplification cascade
• Requires phospholipid surfaces (platelet membrane) and Ca²⁺ ions
• End result: soluble fibrinogen → insoluble fibrin (stable clot)

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Extrinsic Pathway (Tissue Factor Pathway) - FAST pathway

1. TF + Ca²⁺ + Factor VII → TF-VIIa complex (Tenase extrinsic)
2. TF-VIIa activates Factor X → Xa (and also Factor IX → IXa)
3. Factor Xa + Factor Va + Ca²⁺ + phospholipid = Prothrombinase complex
4. Prothrombinase: Prothrombin (II) → Thrombin (IIa)
5. Thrombin: Fibrinogen (I) → Fibrin (Ia) - loose
6. Thrombin also activates Factor XIII
7. Factor XIIIa cross-links fibrin monomers → Stable fibrin clot

• Monitored by Prothrombin Time (PT) - normal: 11-15 seconds
• Used to test extrinsic and common pathway

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Intrinsic Pathway (Contact Activation Pathway) - SLOW pathway

1. Factor XII (Hageman factor) is activated by contact → Factor XIIa
2. Factor XIIa → activates Factor XI → XIa
3. Factor XIa (with Ca²⁺) → activates Factor IX → IXa
4. Factor IXa + Factor VIIIa + Ca²⁺ + phospholipid = Intrinsic Tenase complex
5. This activates Factor X → Xa
6. Then joins common pathway (same as above):
   • Factor Xa + Va → Prothrombinase complex
   • Prothrombin → Thrombin
   • Fibrinogen → Fibrin → Cross-linked stable clot

• Monitored by Activated Partial Thromboplastin Time (aPTT) - normal: 25-35 seconds
• Hemophilia A (Factor VIII deficiency) and Hemophilia B (Factor IX deficiency) affect this pathway

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Common Pathway Summary

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Vitamin K-Dependent Factors

• Warfarin inhibits Vitamin K → blocks extrinsic + common pathway

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⭐⭐⭐ Q3B: Classify Immunity and Add a Note on Cell Mediated Immunity?

Classification of Immunity

IMMUNITY
├── INNATE (Non-specific)
│   ├── Physical barriers: Skin, mucous membranes, cilia
│   ├── Chemical barriers: Lysozyme, HCl, defensins
│   ├── Cellular: Neutrophils, NK cells, macrophages
│   └── Complement system, Interferons
│
└── ADAPTIVE (Specific / Acquired)
    ├── NATURAL
    │   ├── Active: After natural infection (e.g., post-measles)
    │   └── Passive: Maternal antibodies via placenta (IgG) or breast milk (IgA)
    │
    └── ARTIFICIAL
        ├── Active: Vaccination (live attenuated, killed, toxoid, subunit)
        └── Passive: Injection of antisera / immunoglobulins

• Humoral immunity - mediated by B cells and antibodies
• Cell-mediated immunity (CMI) - mediated by T lymphocytes

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Cell-Mediated Immunity (CMI) - Detailed Note

Definition

Components

• CD4+ T helper cells (Th1 subtype primarily)
• CD8+ Cytotoxic T lymphocytes (CTLs)
• Activated macrophages
• Natural Killer (NK) cells

Where Effective

• Intracellular pathogens (Mycobacterium tuberculosis, Listeria, viruses)
• Fungi (Candida, Histoplasma)
• Protozoa (Leishmania)
• Tumor cells (cancer immunosurveillance)
• Transplant rejection (graft rejection)

Mechanism of CMI

• Antigen-presenting cells (APCs: dendritic cells, macrophages) process intracellular antigens
• Peptides are displayed on MHC Class I (recognized by CD8+ T cells) or MHC Class II (recognized by CD4+ T cells)

• T cell receptor (TCR) binds antigen-MHC complex
• Co-stimulatory signal: CD28 on T cell + B7 on APC (Signal 2)
• IL-2 is produced → autocrine proliferation (clonal expansion)

• Secrete IFN-γ → activates macrophages to kill intracellular bacteria
• Secrete IL-2, TNF-β → promote inflammation
• Help CD8+ CTLs

• Recognize antigen on MHC Class I (on infected/tumor cells)
• Kill target cells by:
  • Releasing perforins (pore-forming) and granzymes (trigger apoptosis)
  • Fas-FasL interaction → apoptosis

• Long-lived memory T cells remain after antigen clearance
• Rapid secondary response on re-exposure

Delayed-Type Hypersensitivity (DTH) - Type IV

• A classic example of CMI
• Tuberculin skin test (Mantoux): PPD antigen → CMI reaction at 48-72 hours
• Characterized by macrophage and T cell infiltration

When CMI is Deficient

• HIV/AIDS (CD4+ T cell destruction) → susceptibility to opportunistic infections (TB, PCP, Candida)
• DiGeorge syndrome (thymic aplasia) → absent T cells

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⭐⭐⭐ Q4: Define Anaemia. Classify them and Important Investigations?

Definition

• Men: Hb < 13 g/dL
• Women: Hb < 12 g/dL
• Pregnant women: Hb < 11 g/dL
• Children: Hb < 11 g/dL

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Classification of Anaemia

A. Morphological Classification (Based on MCV and MCH)

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B. Etiological Classification (Based on Cause)

• Acute: Trauma, surgical bleeding
• Chronic: GI bleed, menorrhagia → iron deficiency

• Iron deficiency anemia (most common worldwide)
• Megaloblastic anemia (B12/folate deficiency)
• Aplastic anemia (bone marrow failure)
• Anemia of chronic disease (renal failure, malignancy, chronic infection)

• Intrinsic (corpuscular) defects:
  • Membrane: Hereditary spherocytosis
  • Enzyme: G6PD deficiency, pyruvate kinase deficiency
  • Hemoglobin: Sickle cell anemia, thalassemia
• Extrinsic (extracorpuscular) causes:
  • Autoimmune hemolytic anemia (AIHA)
  • Microangiopathic (TTP, HUS)
  • Malaria, toxins

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Important Investigations for Anaemia

1. Complete Blood Count (CBC)

• Hb concentration (most important)
• RBC count
• Hematocrit (PCV)
• RBC indices: MCV, MCH, MCHC, RDW (Red cell distribution width - elevated in iron deficiency)

2. Peripheral Blood Smear (PBS) - MOST informative

• Cell size: microcytic/macrocytic/normocytic
• Cell color: hypochromic/normochromic
• Cell shape: poikilocytosis (target cells, sickle cells, spherocytes, tear drop cells, schistocytes)
• WBC and platelet morphology

3. Reticulocyte Count

• Normal: 0.5-2.5%
• Elevated in hemolytic anemia, post-hemorrhage (regenerative anemia)
• Low in aplastic anemia, megaloblastic anemia (hypo-regenerative)

4. Iron Studies

5. Bone Marrow Examination

• For aplastic anemia, megaloblastic anemia, leukemia-related anemia
• Hypocellular marrow in aplastic anemia
• Giant megaloblasts and giant metamyelocytes in megaloblastic anemia

6. Specific Tests

• Serum B12 and Folate levels (for megaloblastic anemia)
• Coombs test (Direct/Indirect Antiglobulin Test) - for autoimmune hemolytic anemia
• Hemoglobin electrophoresis - for sickle cell, thalassemia
• G6PD assay - for enzyme deficiency
• Osmotic fragility test - for hereditary spherocytosis
• Stool for occult blood - for GI bleeding
• Urine for hemoglobinuria - for intravascular hemolysis

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⭐ Q5: ABO Blood Grouping and Rh Blood Grouping? A Note on Erythroblastosis Fetalis?

ABO Blood Grouping

Basis

Landsteiner's Law

ABO Blood Groups Table

Nature of ABO Antibodies

• Anti-A and Anti-B are naturally occurring (appear by 6 months of age due to exposure to environmental antigens)
• They are IgM class antibodies (do NOT cross placenta)
• Cause immediate intravascular hemolysis if incompatible blood is transfused

Inheritance

• Controlled by gene on chromosome 9
• Three alleles: Iᴬ, Iᴮ, i
• Iᴬ and Iᴮ are codominant; i is recessive
• Group AB = IᴬIᴮ; Group O = ii

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Rh Blood Grouping

Basis

Key Differences from ABO

• Rh antibodies are NOT naturally occurring - they are formed only after exposure to Rh+ blood (by transfusion or pregnancy)
• Anti-D antibody is IgG class - CAN cross the placenta
• No anti-D is present in an Rh-negative person UNTIL they are first sensitized

Rh Incompatibility in Transfusion

• First transfusion of Rh+ blood to Rh- person: No immediate reaction (no anti-D yet), but sensitization occurs
• Second transfusion: Severe hemolytic transfusion reaction due to anti-D

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Erythroblastosis Fetalis (Hemolytic Disease of the Newborn - HDN)

Definition

Pathophysiology

1. Fetus inherits D antigen from father (Rh+)
2. During delivery, fetal RBCs leak into maternal circulation (fetomaternal hemorrhage)
3. Mother is sensitized - forms anti-D IgG antibodies
4. First baby is usually unaffected

1. Maternal anti-D IgG crosses placenta (IgG can cross - unlike IgM)
2. Anti-D attaches to fetal RBCs
3. Fetal RBCs are destroyed (extravascular hemolysis in fetal spleen)
4. Results in: Severe fetal anemia → compensatory erythropoiesis in liver, spleen → nucleated RBCs (erythroblasts) in blood → "erythroblastosis fetalis"

Clinical Features in Baby

• Hemolytic anemia (varying severity)
• Jaundice (hyperbilirubinemia from RBC breakdown) - may cause Kernicterus (bilirubin deposition in brain → brain damage)
• Hepatosplenomegaly (extramedullary hematopoiesis)
• Hydrops fetalis (severe form - generalized edema due to cardiac failure from severe anemia) - can be fatal
• Nucleated RBCs (erythroblasts) in peripheral blood

Investigations

• Coombs test (Direct): Positive in baby's blood
• Indirect Coombs test: Detects anti-D in mother's serum
• Blood group + Rh typing of mother and father

Treatment

• Phototherapy for neonatal jaundice
• Exchange transfusion in severe cases (replaces sensitized fetal RBCs with Rh-negative blood)
• Intrauterine transfusion for severe in-utero anemia

Prevention (Most Important)

• Rh immunoglobulin (RhoGAM / Anti-D IgG) injection given to Rh-negative mother:
  • At 28 weeks of pregnancy (prophylactic)
  • Within 72 hours after delivery of an Rh-positive baby
  • After any sensitizing event (abortion, amniocentesis, ectopic pregnancy)
• RhoGAM works by binding free fetal Rh+ RBCs in maternal circulation, preventing maternal sensitization

Why ABO incompatibility is less severe

• ABO antibodies are IgM - cannot cross placenta
• Even if ABO IgG occurs, fetal RBCs have weak A/B antigen expression
• Fetal tissues absorb ABO antibodies - protection

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Key Memory Points for Exam:

• Erythropoiesis: 6 stages - Pro → Early N → Intermediate N → Late N → Reticulocyte → RBC; EPO is the main regulator; B12 and Folate for nuclear maturation
• Coagulation: Extrinsic = TF + VII (tested by PT); Intrinsic = XII→XI→IX→VIII (tested by aPTT); both converge at Factor X; Vit K-dependent: II, VII, IX, X
• Immunity: CMI = T cells; kills intracellular bugs, tumor cells, transplant rejection; perforins and granzymes are effector molecules
• Anaemia: Microcytic hypochromic = Iron deficiency; Macrocytic = B12/folate; use reticulocyte count + ferritin to differentiate
• Erythroblastosis fetalis: Rh- mother + Rh+ fetus; anti-D IgG crosses placenta in 2nd pregnancy; prevent with RhoGAM within 72 hours